Violent delinquent Oga encounters a baby one day, which crawls onto his back and immediately forms an attachment to him. Though he doesn't know it yet, this baby is named Kaiser de Emperana Beelzebub IV, or "Baby Beel" for short—the son of the Demon Lord!
Tatsumi Oga, a delinquent student at the worst high school in Japan, arrives at school with a small baby who emits massive shocks of electricity whenever it cries. Oga claims he found after splitting open a strange man who came floating along the river, though his friend Takayuki Furuichi doesn't quite believe. Whilst at Takayuki's house, they are both approached by a demon woman named Hildegarde who claims the baby, known as Kaiser de Emperana Beelzebub IV, is the son of Devil King who was sent to Earth to find a human to raise him so he could destroy humanity.
Hildegarde presents herself to Oga's family in the wrong way, giving them a different view of the situation. She also informs Oga that if he is more than 15 meters away from Beelzebub, his cries may destroy the city. Whilst dealing with this situation, Oga comes up with the idea that, if he can find someone more strong and ruthless than him, he can dump Beelzebub on him and be free.
When Oga loses some food for Beelzebub and he starts to get hungry, Oga instead uses up a vending machine's worth of yoghurt drink, revealed to be the favourite of the second year delinquent, Hajime Kanzaki. Figuring he may be worthy of dumping Beelzebub on, Oga goes to see him under the pretense of becoming his henchman. After beating his current henchman, Oga is asked by Hajime to throw him out of the window, but instead decides he isn't worthy based on Beelzebub's expression and knocks him out instead.
Beelzebub is currently in a phase of non-stop urination that threatens to flood the city. Hildegarde manages to temporarily stop it with an interdimensional diaper, but in order to stop it completely, Oga decides he needs to get a super absorbent daiper advertised on telly. Upon reaching the supermarket only to find the brand had already sold out, it is taking hostage by a group of thugs. As Oga probes everyone to see how ruthless they are, Beelzebub's diaper gets stuck on the leader's gun. Whilst the leader considers raising Beelzebub, Oga ends up changing his mind and beats him, before they decide to hover Beelzebub over the ocean to finish urinating.
After a sleep deprived night, Oga notices he has a strange mark on his hand, revealed to be Zebel Spell, a symbol of the contract between him and Beelzebub. Hildegarde and Alaindelon explain that a Demon Lord's parent is a vessel for his power, and the mark will grow the more opponents Oga defeats. Wanting to get rid of the mark, Oga decides to give up fighting in the hopes it'll make the mark smaller. Meanwhile, another delinquent, Tatsuya Himekawa, hires some thugs to kidnap Hildegarde and Takayuki in order to get Oga's attention. Oga uses Alaindelon to sneak in and forgoes his promise to beat Tatsuya's henchmen. Tatsuya goads Oga into hitting a cermanic plate hidden under his shirt, but his punch is so powerful it knocks him out anyway. Tatsuya then tries using a stun rod, but Oga resists it and uses Beelzebub's power to beat him, which in turn makes his mark larger.
The mark on Oga's arm goes back to its small size, much to the chagrin of Hildegarde who destroys his games console. She brings a play room for Beelzebub to play in, but upon building it, it turns out to be a magical cage that both she and Oga get stuck inside. The only way to get out is to have Beelzebub solve a block puzzle toy, but he just ends up breaking it. Frustrated, Oga utilises his Zebel power to punch his way out, though it makes his mark grow big again. Rummaging through a bag of demon world toys, Beelzebub opens a chest that transforms Oga and Takayuki into nurses, which can only be reverted by helping Beelzebub play Doctor. Oga decides to use the opportunity to try and do good deeds and get rid of his mark, though has trouble handling the demonic medical supplies. After some failed attempts at treating some of the students, they change back after discovering an actually useful tool on Beelzebub's costume, but accidentally triggers an explosive that negates all the work Oga did getting his mark down to size.
Oga's family coerce him into taking Beelzebub to the park so he can make his 'park debut'. Despite making a bad impression with the other mothers, Oga meets a girl named Aoi who is looking after her little brother, Kouta. Aoi tries to help Oga make his park debut which, partly due to the appearance of Alaindelon, doesn't reap much success. The general antics convince the other mothers to call over a police officer, who starts badmouthing both Oga and Aoi. Before Aoi can lose her temper, Oga throws him into a trash can, with Aoi coming off impressed. After Oga leaves, it is revealed Aoi's true identity is that of Aoi Kunieda, leader of the Red Tails gang and one of the Tōhōshinki.
Aoi and the Red Tails return to Ishiyama High to deal with Oga, despite not knowing who he is. When Takayuki drags Oga to meet her, Aoi is shocked to find Oga is the one she met at the park, although he doesn't recognize her back. Aoi starts to fight him using powerful bokuto attacks that cut through glass, but he manages to dodge all of her attacks. Impressed by her strength, Oga asks Aoi to become Beelzebub's mother which causes her to become embarrassed and run off. Afterward, Aoi seems to develop feelings towards Oga. Meanwhile, another delinquent gang plots to take down Aoi.
The delinquent group, MK-5, come to confront Oga, but are promptly beaten by him and Hildegarde, who catches the attention of a smitten Aoi. As Aoi's right hand ladies, Chiaki Tanimura and Nene Oomura confront Oga, Hildegarde, who had heard from Oga about his plans to dump Beelzebub onto Aoi, challenges her to a fight to test her strength. When Aoi returns to find Chiaki and Nene injured, she assumes Oga is responsible and confronts him in a fight. In actuality, Chiaki and Nene were attacked by MK-5, under the orders of Miwa, who desires the title of Queen. Shintarou helps Nene get to Aoi to tell her the truth, but when Miwa makes his move, Oga promptly beats him up. Afterward, Aoi is once again baffled by Oga trying to give her Beelzebub, explaining that he's forced to raise him because of Hildegarde. Later, Aoi decides to quit the Red Tails and put Nene in charge, feeling that she is no longer able to serve as their leader.
A stray cat becomes attached to Oga, causing Beelzebub to become extremely jealous and shock anyone he shows affection towards. Beelzebub challenges the cat to a fight and subsequently loses, but when a gang of fierce cats gang up on the cat for fighting in their territory, Beelzebub stands up for him, prompting Oga to help out. Afterward, Beelzebub and the cat become friends. Later, Oga takes Beelzebub and the cat to a play center where he once again meets Aoi in her park getup. While Beelzebub and Kota getting into heated competition with some other toddlers, Oga and Aoi have a conversation full of misunderstandings, both ending with explosive results. Afterward, the cat decides to leave in order to protect his fellow strays.
Himekawa plots revenge against Oga by setting him up with a poisoned sandwich, but it doesn't affect him due to his resilience to Hilda's cooking. He next attempts to use a popular costumed character in order to try and capture Beelzebub, but Oga and Beelzebub notice that he's a fake due to differently colored chopsticks. He later attempts to seduce Takayuki into giving him Beelzebub, but Alaindelon intervenes, transporting Himekawa and his gang to a far off island. The resulting failures leave Himekawa with no money to hire any more henchmen. Ans as he tries to attack Oga by himself, he gets caught up in fighting against a rival gang also attempting to kill Oga. Afterward, Kanzaki offers him one of his yogurt drinks, to which Himekawa repays him with a year's supply with a month long lifespan.
Beelzebub has been crying all night for the past three nights, which is causing Oga and the whole neighborhood to get very little sleep. Wanting to stop his night crying to avoid being chased out of their house, the Oga family attempt to play with Beelzebub all day in order to tire him out come nighttime. Despite managing to get him to sleep, Beelzebub still wakes up and cries in the middle of the night. After Oga spends the day dealing with a sleep deprived Tōhōshinki, Hilda brings a toy that calms Beelzebub down, but is even noisier than his crying.
As Ishiyama High holds an assessment test for its studyphobic students, the Great Demon Lord also holds a test for Beelzebub, requiring him to earn points by beating up various things, or run the risk of both he and Oga facing remedial classes in the Demon World. With Beelzebub not showing much hope, Oga decides to try and dump Beelzebub on whoever scores highest on the test. When a rival school interrupts the test, Oga fights them off to allow the other students to keep on the test, earning enough points to avoid remedial lessons, later learning that the smartest student it actually Takayuki. Later, Hilda prepares a letter to the Great Demon Lord, writing reports on Oga, his family and the Tōhōshinki.
As Kanzaki tries to come up with a new special move to beat Oga, his followers suggest the key to Oga's power is having Beelzebub on his back. Thus, Kanzaki starts training with a weighted baby doll on his back. When that plan doesn't work, they try finding a living substitute, which end in more embarrassing results. Meanwhile, Oga takes Beelzebub to the park where they once again meet Aoi, who has lost track of Kouta. Kanzaki coincidentally finds him and deems him a worthy substitute for Beelzebub. Whilst playing with him, he figures out the key to performing his new special move. After returning Kouta to Aoi, Kanzaki performs his special move, though it just leaves everyone confused. As Kanzaki leaves, Oga admits his move was surprising.
Takayuki tells Oga about the final member of the Tōhōshinki, Hidetora Tōjō, mainly to get him interested about ditching Beelzebub so Alaidelon will leave him alone. Wanting Oga to get beaten, Himekawa informs them that Tōjō works at the beach. Oga and Tatsumi go there the next day and get mistaken as part-timers for a yakisoba stand, later finding that Tōjō had actually been fire the previous night. As he keeps missing him, Oga asks Aoi to pass on a letter of challenge to Tōjō, whose henchmen find it before Aoi can rip it up. Oga gets held up to their meeting, arriving only finding a pile of beat up gang members, which Tōjō beat all by himself. Despite Aoi's warnings, Oga looks forward to finally meeting Tōjō.
Takayuki is dragged away from his tropical summer vacation against his will to join Oga and Beelzebub to a public pool, though Oga is soon shocked by the fact he could've gone to the tropical resort himself. Whilst chatting with the Red Tails, Takayuki is approached by some delinquents from his old middle school and ends up going against their leader, Takashima, when they start hitting on the girls. Takayuki tries to hide in the pool, but Takashima calls more of his men and tricks the lifeguard into evacuating the pool, leaving Takayuki and a stunned Oga. As Takayuki becomes surrounded, he forces Beelzebub to cry in order to shock all the delinquents before facing Oga's wrath.
The Demon King sends Beelzebub some summer homework, containing a notebook which brings anything drawn in it to life. Next, Hilda brings Oga to Ishiyama High to collect prints of rare, powerful beasts, aka the delinquents, by whacking them with a giant fly swatter. During the chase, Beelzebub ends up planting several carnivorous Demon Plants, one of which grows into a massive giant causing havoc until Oga destroys it.
Beelzebub gets a summer cold, causing his body to give off incredible heat. As his condition gets worse, the Zebel Spell on Oga's hand disappears, leading Hilda to become frustrated towards Oga for showing little care towards him. As Oga tries to decide what to do, he encounters one of Tojo's henchmen, Shojo Aizawa, who seemingly outmatches Oga before leaving. The next morning, both Hilda and Beelzebub have disappeared. As Oga searches for them, he encounters Tojo with Beelzebub on his back, who challenges Oga to a fight. As the two fight, Oga is shocked to see what appears to be the Zebel Spell on Tojo's shoulder, before he is knocked clean into the river.
Oga returns home after being defeated by Tojo to find that Hilda has returned from the Demon World with the imperial doctor, Forcas Rachmaninoff, and his assistant, Lamia, to help determine the cause of Beelzebub's illness. He diagnoses the illness as King Fever and when Beelzebub was trying to draw out massive amounts of demonic power, he accidentally severed the link between him and Oga and found Tojo similar to Oga and connects to him. Oga begins a treatment to try and reconnect with Beelzebub and enters a dream world through the effects of a Demon World drug that is shot into his head where he eventually finds a sleeping Beelzebub in the middle of several inscribed circles that are shrouded in an electrical field. After about 6 hours of treatment, Oga still has not waken up and Lamia assumes that he has died and becomes happy. Oga suddenly wakes up and spanks Lamia and claims he is teaching her some manners. Hilda asks Oga how he feels after having gone through the hard part of reconnecting with Beelzebub and Oga says he is in the mood to get Beelzebub back.
Oga reflects on Lamia telling him to take a shower before going to Tojo and sees a silhouette outside his bathroom window and thinks it's Lamia, but it's really Shiroyama, who takes Oga to Kanzaki and Himekawa who tell him they want to team up to beat Tojo and Oga agrees. On the way to Ishiyama High, they are surrounded by a large group of delinquents, which Oga takes care of swiftly. Upon arriving at Ishiyama High, the rest of the school is there to confront Oga, only to have Kunieda and the Red Tails take care of them for him. Tojo sets off fireworks in order to try and cheer up Beelzebub and all the delinquents are captivated by them. Oga confronts Tojo and Tojo is willing to give Beelzebub back only to have Oga tell him that whoever Beelzebub wants to stay with is up to him. Beelzebub's fever breaks and he immediately goes to Oga and climbs on his head, and a flashback shows Oga reconnecting the bond between him and Beelzebub. Just as the episode ends, Oga and Tojo are about to begin their fight.
Furuichi is woken up by Alaindelon at 4:00 A.M. and is teleported to Ishiyama High to watch Oga and Tojo's fight with Hilda, Lamia, and Forcas Rachmaninoff. Meanwhile, Kanzaki, Himekawa, and Shiroyama confront Aizawa and Karou and begin to fight, but are quickly beaten by them, but they refuse to give up because of their stubborness. Natsume shows up and defeats Karou in one blow, and is about to deal with Aizawa. Kunieda and the Red Tails finish dealing with the delinquents and go to watch Oga and Tojo's fight, with Oga at a disadvantage. Beelzebub activates the Zebul Spell, thinking Oga cannot win, shocking everyone. Lamia accidentally sets off one remaining firework and it's aimed at Oga and Tojo. Oga easily stops the firework and causes a giant flaming Zebul Spell to appear on the ground. Oga orders Beelzebub to not interfere in his fight, promising he will win and Beelzebub erases the Zebul Spell. Oga and Tojo resume their fight, which goes on until dawn, and just as it looks like Tojo is about to win, Oga spins around and performs a suplex on Tojo, winning the fight. Oga tells Beelzebub he won the fight and Beelzebub tackles Oga to the ground. Tojo's past is revealed, showing a man he looked up to who taught Tojo the ture meaning of fighting and had a Zebul Spell on his shoulder, which Tojo got tattooed on his shoulder in order to look like him, and Tojo tells Oga he is now the strongest at Ishiyama High.
Baby Beel's fear of insects prompts Oga to give him some special training up in the mountainous village of Mapputatsu. There, he stumbles across Kunieda carrying Kōta in the disguise she wore at the park with her grandfather. While Oga is still unaware of her true identity as Kunieda, her grandfather develops an interest in him.
As the Zebub Spell is restored, Lamia and Dr. Forcas begins to return to the Demon World through Alaindelon. While they were being transported, Baby Beel grabbed onto Lamia and refused to let go, causing Furuichi and Oga to be transported as well. All 4 of them landed in Vlad's Haunt, one of the most dangerous areas in the Demon World, with Alaindelon unconscious. Lamia manages to contact Hilda through her cellphone, and is informed that Alaindelon's daughter might be able to transport them home safely. A Yople alien shows up, which Oga punches, causing the Yople to call the creatures in Vlad's Haunt to attack the group. As the group escapes to find Alaindelon's daughter, they encounter numerous creatures, with Furuichi suffering the worst of their attacks. When the group finally reaches their destination, they realise the place was ransacked by bandits, Gare and Edda, who have kidnapped Alaindelon's daughter. As Oga defeats the bandits easily, the episode ends with Furuichi realising how attractive the daughter is.
Finding Alaindelon's daughter rather attractive, Furuichi vows to rescue her while Gare and Edda leaded him to Oga and Lamia in the village of thieves where Alaindelon's daughter, Angelica is trapped. The rescue runs smoothly until a large Demon named Vlad's Master is awakened and terrorizes the village in which Beelzebub suddenly growed bigger and larger to fight it off.
The students of Ishiyama High settle in at St. Ishiyama, with all the significant characters being placed in the same classroom. Their new homeroom teacher, Sadohara, challenges Tojo to an arm wrestle to try to establish some authority over the class, failing miserably when Tojo accidentally slams him upside-down against the wall. He then picks a second target, Oga, who was sleeping in his seat, and wakes Oga in order to challenge him. Baby Beel is angry to have been woken and subsequently tantrums, shocking Oga and Sudohara.
Oga and Furuichi accept Kazuya as their student, after being praised on how legendary they are. Kazuya tries to copy Oga to become strong like him, even making identical lunch boxes. Due to the Teimo incident, Oga and Kunieda meet Kido, the teacher in charge of student life at Saint Ishiyama Academy. They receive a warning of expulsion, should they persist to cause trouble for the school. Meanwhile, Kunieda tries to learn from Azusa to be more open to men, but realises it to be impossible. In order to get Kazuya off his back, Oga pretends to teach the secret technique of electric shocks, and manages to convince Kazuya successfully. Kazuya hears news of Azusa being kidnapped by a bald man suspected to be a Teimo member, and tries to rescue her with his new "powers". After Kazuya fails to land any hits, Kunieda arrives and reveals the man to be an acquaintance of her temple, and the situation is resolved.
In order to make up for forgetting his daughter's birthday, Alaindelon brought Angelica to Furuichi's house for a tour of the Human World. Unfortunately while the gifts were being retrieved from Alaindelon, a Yople alien teleported into the human world and escaped. Angelica resolves to track down the Yople, using a demon wand that transforms its user into a Magical Girl. Gathering 4 other girls - Hilda, Azusa, Honoka and Aoi, they form the 5 Fairies of Light and manages to track down the Yople at an amusement park where Tojo was working as part of the cast in a Gohan-kun theatre drama. A rival girl gang challenges Aoi while they were capturing the Yople and a fight ensures. Eventually, Furuichi manages to pin down the Yople and both of them are teleported away with Angelica's ability. Furuichi eventually returns back to the human world through Alaindelon, leaving him to wonder if he was even inside Angelica at all in the first place. Unfortunately, Angelica left before Furuichi could ask her about it.
The students of Ishiyama High are ordered to do physical examinations amongst themselves. Baby Beel gets jealous of a toy which accidentally gets stuck to Oga's back, and locks himself inside of a locker to pout in protest. While trying to lure him out, the girls come into the room for their physicals, forcing Oga and Furuichi to hide under the desk while the girls strip off and start measuring. The girls leave the room without seeing them, despite a few close calls, and upon the girls' departure Oga and Furuichi get up from under the table and start trying to lure Baby Beel out once more. Aoi returns to the classroom moments after leaving, and finds the two of them in the room. She realizes that they have been there all along, and goes on the offensive. During the conflict Baby Beel exits the classroom through an open window and climbs into Sadohara's sports car, and locks the door behind him, which Oga is unable to pull open. Baby Beel "springs a leak", filling the car. Oga, unable to think of any other option, chops the car resulting in the entire shell of the car shattering.
After being shocked early in the morning by Baby Beel, Oga's enthusiasm to find someone new to pass Beel onto is temporarily reborn, and upon hearing rumors of six students who excel both academically and athletically, he goes off searching for these six in the hopes that they will impress Baby Beel. Oga, accompanied by Aoi, goes searching for Kazuya to get more information about the 6 Holy Knights. Meanwhile, Furuichi runs into an old friend, Miki. He warns Furuichi to stop Oga getting involved with the six, and tells him that the six wear badges to identify them as Holy Knights, before promptly revealing himself to be one of them. Oga finds Azusa while looking for Kazuya, and is quickly interrupted when two of the Knights show up, revealing themselves to be Alex and Sakaki. Sakaki tests Aoi's strength first by attacking her with a wooden sword, while she only has a ruler she borrowed from Azusa to defend herself with. Aoi cuts the tip of Sakaki's sword off during his attack, proving herself to be the more skilled swordsman. Oga refuses to fight his opponent, Alex, and instead quizzes him while attempting to dodge his attacks. Alex gives up when he realizes Oga is not taking the fight seriously, and the two knights leave the scene.
Furuichi and Oga survey the school from the roof searching for emblems on people's shirts, and Azusa ends up tagging along in their search. Two students from St. Ishiyama are seen hassling two of Aoi's gang, causing Shiroyama to interject, telling them to leave. The two disgruntled students are offered a free punch by Shiroyama as compensation for breaking off their advance on the girls. They roll a large weight onto Shiroyama, hospitalizing him. Kanzaki bursts into the classroom the two boys are in later that day to get vengeance, only to be knocked out by Miki in one blow. Miki the invites the students of Ishiyama High to meet him on the roof after school. Himekawa, Natsume, Furuichi, Oga and Aoi all go to the roof after school, though Furuichi stays in hiding.
The fight between Oga and Miki begins, with Miki seemingly beating Oga with ease. He strikes Oga with a technique which Aoi recognizes to be Hakkyokuken, a martial art form from China. Oga continues to fail to hurt Miki, using a variety of "special moves" which don't seem any different from his usual attacks. Miki begins to power up his most powerful technique, which he advises Oga to dodge due to its power. Before he has a chance to use it, he is interrupted by Nanami, the 2nd strongest of the 6 Holy Knights. She is shortly joined by the final knight, Izuma, the strongest of the six. Aoi senses how much power he has and recognizes that they have no hope of defeating him. The students return to their classroom to find a notice, declaring all of the Ishiyama High students who were present on the rooftop expelled. The students go to the Counselor's office to complain, and are told that they have a chance to avoid expulsion - by beating the 6 Holy Knights at a sporting event at the upcoming school festival. The students reluctantly accept and leave the office.
Hilda transfers into the class to keep an eye on how things are progressing, with Alaindelon transferring into another class. The students find out the sport will be volleyball, something none of them have any experience in. The students lack motivation which Hilda promises to sort out. With the help of Alaindelon they kidnap the delinquents and force them to watch a poorly edited video of the 6 Holy Knights bad-mouthing them. The delinquents, except Furuichi, all believe in the video and become fired up, ready to beat their opponents.
Oga becomes tired of practicing and walks out of the gym, itching for a fight. He finds Miki training in a nearby Dojo, and challenges him to a fight. Oga takes all of Miki's attacks head on, and is clearly damaged from the strength of the attacks. Miki claims his technique is only at 70% power, but will be ready by the time the school festival arrives. Miki has a flashback to a time when Oga betrayed him in the past, where Miki followed Oga to fight alongside him, but Oga turned on him, knocking him down and kicking him on the floor. Oga finds Tojo and challenges him to a fight, looking for some insight into what his own special technique should be. Oga appears to be quite beaten up after fighting Tojo, but not deterred from it at all. He claims that it is all a matter of fighting spirit, and that he is ready to win at volleyball and any fights he gets into.
The festival is underway, and Azusa is trying to run a Maid café. Furuicihi offers to help, and Azusa and her friends obey his commands blindly due to his reputation as 'The Strategist'. Furuichi persuades many of the Red Tails to dress up in Maid uniforms, claiming that this is a preliminary battle against a café that the 6 Holy Knights are running elsewhere. Hilda also gets involved, and remodels the café to be more formal. Some rowdy guests from Teimo High enter the cafe acting unruly, whom Hilda is quick to discipline.
The match between Ishiyama and the 6 Holy Knights is here, and this is not your usual volleyball! With their pride and their educational future on the line, the Ishiyama gang puts it all out there on the court! Will Furuichi the Strategist be their trump card?
Kiriya, the man who scarred Miki and tried to attack Oga in the past shows up at the volleyball match. He declares that he is seeking revenge on Oga, and has taken hostages at the school to stop anyone interfering. Kiriya reveals that he had to move away to Nara previously, and this is why he is only now seeking revenge on Oga. Miki, who also moved to Nara after being beaten by Oga, realizes that Oga had only beaten him to protect him from Kiriya when the two of them were both in Nara. Oga and Miki team up to take down Kiriya's henchmen while the 6 Holy Knights, the Red tails and the Tōhōshinki incapacitated all of the Keimo High students holding the St. Ishiyama students captive.
Baby Be'el is having a great time at a railroad crossing and wishes Oga to sing with him. Oga, however, is thinking about the Izuma's words about demon powers. Aoi, who is happens to be behind them, runs into Nene and Yuka, and they once again assume Aoi is going to confess to Oga. At school, Oga has become a celebrity for the "magic trick" after the volleyball match. Furichi drags Oga into creating another illusion a.k.a. making Furichi disappear by sending him flying using his mindless violence. Aoi, not convinced, confronts Izuma; however, she doesn't make any progress. Later, Tōjō talks to Shizuka stating his fight with Izuma isn't over. The two meet on the roof, and Oga joins in. However, before they can begin, Saotome beats them up and brings the unconscious Oga and Toujou to clsss.
Oga attempts to explain the whole situation to Aoi, but him and Furuichi get stuck on remember BabyBeel's name. Not long after, a demon maid servent who looks similar to Hilda named Yolda appears, and expresses her unhappiness on the situation. Soon another 2 demon maid servents appear along with a young boy who is introduced by the maids as Lord En, BabyBeel's older brother, who has come due to BabyBeel taking too long to destroy humanity. Aoi then understands everyone that's going on. BabyBeel and Lord En do not have the best relationship, where BabyBeel is constantly ignoring the attention-seeking Lord En, making him upset. Lord En is also seen to love anything to do with games.
Before destroying humanity, Lord En wants to experience everything in the human world. As Lord En is a huge fan of games and the like, he is drawn to the game centres and amusement parks there and is distracted from his original mission of destroying humanity. During the visit to Ishiyama Land it's found that he cries very easily over the slightest things that BabyBeel does or happens in general. He suggests going on a roller coaster, which everyone is against due to his crying, but still he demands to go on. He does eventually end up crying and creating fire once again on the roller coaster. Lord En then alerts Oga that he may be attacked by his private guards.
It's revealed that Tojo and Suzuka know each other from the past, as they both underwent training from Saotome. Izumo and Tojo have a fight after school and Izumo reveals some of his demon powers. However Behemoth's 34 Pillar Divison has arrived and causes trouble. Aoi is captured by one of the pillars named Hecatos who wants to make her his contractor so that he can use all his powers in the human world. Oga and Hilda then fight together against him in order to rescue Aoi, however just before they believed they'd won Hilda is stabbed by Hecatos.
Whilst everything is in commotion, Lord En continues his tour of the human world and is now in Akihabara. As usual, he also cries due the maids being mistaken for cosplayers and he feels left out. Also, Furuichi is called a pervert by Alaindelon's daughter. Kanzaki and Yuka are both spying on Tojo and Izuma's fight, until two of the pillars: Graffel and Naga interrupt the fight and beat Izuma up. Izuma is revealed to be only half demon and hasn't actually made contact with any real demons before and so his power is weaker than that of a real one. Tojo then cuts in and so does Kanzaki however Graffel easily defeats Kanzaki, and Tojo fights till he is unconscious, however they soon leave to join Hecatos. Oga attempts to beat Hecatos using his Zebel Blast, however is blocked easily. Saotome then enters and uses his spell master magic forcing Graffel, Naga and Hecatos to retreat. Quickly, they call Lamia and Dr. Bloo to check on Hilda before it's too late.
Hilda recovers and apologizes for everything and being unable to help. Oga decides to become stronger in order to protect everyone and so seeks for Aoi's grandfather to train him. Oga's younger sister is revealed to be a member of the Red Tails. After seeing Kouta come out of the shrine Oga realises that Aoi Kuni is actually Aoi's cousin. Oga and Teima Shadow receives pointless training from Aoi's grandfather. Saotome and Tojo talk about the incident, however Tojo doesn't understand everything about demons and mistakes them for bears and Saotome's friends.
Lamia asks Fruichi to find Baby Beel's brother but when they went to the game center, they find Kanzaki,Himekawa, and Yuko. Furuichi asks them for help. Meanwhile Aoi and Oga went for training to be strong.
Oga begins his training with Ittousai while Furuichi and Lamia enlists the help of their Ishiyama classmates to search for Lord En. Can Oga and Beel complete their training in time and will Furuichi and Lamia find Lord En before he destroys the city with his army?
Furuichi's group go to Himekawa's home, to start searching for En in online games. When they arrive they find the place is a giant skyscraper building, with several rooms dedicated to gaming. Most of the group have never played games, except for Chiaki, who immediately proceeds to find En in an online fighting game. Furuichi tries to get him to tell them where he is, and when he finally answers he tells them they have to beat him in a game before he tells them. Tatsumi tries to escape from Saotome, but is unable to throw his teacher off, even after showing off his new rock splitting abilities. It isn't until later in the episode that Beel takes matters into his own hands and decides to let Saotome train him. With Saotome training Tatsumi, the group's first teacher substitues for them, but everyone is away doing their own thing, except for the girl with the face mask, who for some reason got left behind.
The episode opens with a distinct sci-fi feel, before moving into the game, where the Ishiyama group are in the middle of getting their asses handed to them. They are unable to do anything against the tank. Then Yuka shows up, driving a giant robot called the Crazy Frog. She crushes the tank, and proceeds to kick and tap dance all over the other vehicles. Up until she steps on a mine and loses the tank. Tatsumi's training goes more like an aerobic workout, complete with lame musical accompaniment. He complains about this, calling out for being an exercise, and not training. Meanwhile the gamers run into more trouble when En's team start using an invisibility cheat. Himekawa realizes what they are doing, and goes to prepare some countermeasures. While Tatsumi does pushups, Saotome tells him that Tojo asked to be trained, and if he isn't careful Tatsumi will be overtaken. Saotome uses a recording of Tatsumi's voice to make a training partner who has all his hidden strength.
The show opens seeing the 2 pillars who got beaten in jail, and 2 other pillars talking to them. One is Lamia's mom. They then cut to the main gang who are tired from playing video games. Shiro acts as Lamia while talking to En. He speaks like a ditzy schoolgirl, but En buys it.
After Oga came back from his training he introduce his new technique called the "Zebul Emblem" and "Super Milk Time" (He learn it while he was training with Saotome and Black Baby Beel and Baby Beel is fighting over the milk and Oga Decide to drink it himself so Black Baby-Beel And Baby-Beel doesn't have anything to fight on) to remove the divider between Oga and Baby-Beel. Oga suddenly became powerful after drinking the milk and the demon power from Baby-Beel increase and Oga defeat the two Pillar heads and after that he became unconscious and the Pillar Head Naga also fight Oga and Oga drink more of the milk then He and Baby-Beel merge and they became more powerful Oga Grew a wing and Scattered the Zebul Emblem to surround the Pillar Heads and at last he defeated the three Pillar Heads and the top of Himekawa's hotel was destroyed due to the explosion.
A tired and thirsty Furuichi finds himself in a desert, accompanied by Oga, Baby Beel and Kunieda. Furuichi asks what the hell is happening but Oga replies that this is Journey to the West, that he is the monk Sanzohoshi and they are traveling to Tenjiku (India) to retrieve the Buddhist Scrolls. Before dusk, Oga and company arrive at a village populated by people who look like Shiroyama. The townspeople prepared dinner for the four of them. They continued their journey, with more and more people accompanying them along the way, first the Six Holy Knights, then the Red Tails, then the MK5, until the whole of Ishiyama accompanied them to the doors of the Ox King's castle. They were surprised, however, to find Lord En and his three maidservants inside. En tries to hold a New Year's party for his newfound "friends" (Isabella comments that En does not have friends who visit him at New Year) by playing a card game. However, Furuichi beats En at the game, causing the latter to cry and set the Ox King's palace on fire. They later find out that the Ox King is not the Demon Lord himself, but Tojo, and he steals all of the octopuses from the village in order to make Takoyaki, as it was his part-time job. Oga borrows Tojo's fan and tries to quell the flames, but to no effect until Tojo comes with the real Bassho Fan and extinguishes the flame. After chiding En being a pain in the ass, Tojo invites everyone present to a Takoyaki party, at which point Oga and Baby Beel forgets totally forgets about their quest and joins in eating.
Due to the after effects of Super Milk Time, Baby Beel and Oga have swapped bodies. In order to return things back to normal, they try to look for Saotome-Sensei to see if he knows how to, however they are continuously disrupted by everyone else from the academy trying to start a fight with Oga (who is currently Baby Beel). Eventually, Baby Beel runs away after being drawn to a smell, and Hilda, Furuichi and Oga try to catch him, where they find him attached to Aoi. After hugging Aoi, BabyBeel runs off once again and so Hilda and Aoi have a match to see who can catch Baby Beel. Although at the end BabyBeel chooses Aoi over Hilda after she shows some of her new demon magic from training. Not long after, Aoi and Hilda attack Oga(Baby Beel) when fighting over him, and just before they hit him Oga and BabyBeel return to normal, leaving Oga being severely hurt by the two and BabyBeel perfectly fine. Saotome-sensei then comes to school the day after.
Oga is trying to think of a way to perfect Super Milk Time before they're attacked again whilst Nene and the rest of the Red Tails are trying to get Aoi back into the group by holding a fake meeting at Aoi's place and so persuading her. Aoi's demon magic is revealed to be thanks to Koma, a perverted demon whom she acquired at Mapputatsu Temple. Oga has found a new milk bottle which will hopefully help Super Milk Time and is asking Furuichi to try it, who is obviously reluctant as he is "busy". Meanwhile, St Marian's High School for Girls have declared a battle against the Red Tails in order to overtake the Southern Kanto region as well as the North, however they are unknowingly defeated by Koma who runs around touching each of them and so making them feel weak and uncomfortable therefore being unable to fight.
It's the coldest day of Winter and St.Ishiyama is closed, however everyone from Ishiyama has still come to school as they were not informed of this until Kazuya and Azusa come in to tell them. Unable to go home due to the storm, everyone remains in school, until suddenly the delinquents slowly get beaten up - Kazuya then informs the others that the incidents are exactly the same as that in the counting song "100 Delinquents". As the song's events continues, everyone decides to try and capture the culprit however all the men are eventually beat up leaving only the women remaining. Aoi then realises that the culprit is actually Koma, who wanted the women to warm each other up naked in the cold, whilst he stayed in the middle - and so beats him up.
Oga ,Furuichi and their teacher (which was the Spellmaster) was searching for the black version of Baby Beel. Oga thought that it was so bored that it ran away. But actually it was because of Himekawa.They asked some girls if they saw someone with a baby, and they answered that they saw a nice looking, hot guy that smelled so nice. He took it because he believes that it was the key for their family to be more successful. In the end, Black Baby Beel was reunited with Oga's teacher.
When Black Beel has gone missing, Zenjuro urges Oga, Furuichi, and Beel to look for him before something bad happens. Meanwhile, Himekawa who has kidnapped Black Beel for his close resemblance to a lucky statue experiences great luck and wealth. Somehow, Black Beel has grown gigantic.
Himekawa introduces himself and tells everyone how he makes his "super cool" pompadour: by hardening it with a hair gel called Pomade. Furuichi ignores him and mentions a man showing up. Himekawa thinks he's referring to the Legendary Warrior, Super Pompadour. Oga asks who that is. After Himekawa introduces the two-part episode, Oga becomes surprised that the Great Demon Lord is showing up, while Himekawa ignores him and worries about his lost pompadour.
Hilda notes that Oga and his friends are acting funny and that they're scheming something bad. Oga and Beel deny this, but do so in such a poor manner that Hilda easily notices they're lying. After Oga introduces the next episode, Hilda begins to think of a new dance for Baby Beel in place of the Pudding Pudding dance. Oga responds with a fake laugh.
Koma gets an idea from Oga's Super Milk Time training to switch bodies with Aoi Kunieda by offering her Manju. He has some fun moments with Aoi that gets pervy but he also spends a good bit of time with the kids, of which Koma fits in pretty well. His attempts to push them into his own designs are definitely fun since he wants to utilize the body switching ability for his own ends, which is essentially to get into Aoi’s body and play with her end. Hilda wins a ticket to an Onsen where Oga's family and her enjoy a relaxing bath. Oga, Beel, and Hilda bond a little bit while Furuichi thinks he has a chance to see a woman naked in the mixed hot spring bath.
Behemoth visits St. Ishiyama to find Oga and Beelzebub, but he gets welcomed by Zenjuro, Ittousai, and the principal. Hilda holds Oga down while they watch the fight. Meanwhile, Furuichi and the delinquents arrive at Akumano Academy. They stumble upon the Pillar Generals. Agiel attacks only to meet Aoi who arrived in the nick of time. Will Aoi defeat Agiel?
While Aoi fights Agiel, Oga and Hilda meet Behemoth and Jabberwock up close and personal. Then, at Akumano Academy, Lord En and quetzalcoatl has the delinquents playing a "game," and at the same time, Oga fights Jabberwock to free Hilda from the enormous jaws of Sodom. Will Oga and Beel have the power to free Hilda from Jabberwock and his dragon, Sodom?
It begins with a bunch of Ishiyama delinquents crying and mourning over the loss of Oga and Baby Beel. Their tears stop short by the river when Alaindelon cruises like a speedboat and Oga and Beel pop out of the man. At Oga's house, Hilda explains that the Great Demon Lord has no interest in destroying humanity which means the goal of raising Beel to destroy all humanity is no more. Beel and his gang will have to go home, and mass hysteria follows. Will Oga, Beel, and the gang part ways forever?
