"Monstar" depicts the lives of ordinary teens who are injured psychologically and heal themselves through the power of music. Yoon Sul-Chan is the vocalist for the group "Man in Black". He carries deep wounds within his mind, but doesn't express his feelings. Because of his hot temper, he causes trouble. His management company then orders him to carry himself as an ideal student. He joins a music band at school, while facing unexpected events.
While filming a movie in a high school, idol-actor Yoon Sul Chan from the fictional group Man In Black becomes involved in a scandal. Jeopardizing MIB’s new album release, his label company decides to send him to school for 3 months.
As the second new transfer student of the school, Yoon Sul Chan has no choice but to sit next to Min Se Yi, whom has developed a strong dislike towards him, from an event that occurred in the first episode. All For One bluntly rejects Teacher Dok Go's request to play at a charity event, and we're introduced to 'Adam', a mysterious man in the park that seems to know the song MSY sang out loud.
Excluded from his assigned music group, in order to earn his grade solo, Yoon Sul Chan plays the song he heard Min Se Yi perform in the park, replacing them with his own comical lyrics. After an event in the shrines, the involving students are called to the office and forced to come up with something to perform at the charity event or have their reputation tarnished.
We have already seen that there is a history between Yoon Sul Chan and Min Se Yi, Yoon Sul Chan and Jung Sun Woo, the person we currently know as "Adam" and Min Se Yi, and (last but not least) "Radio" and Cha Do-Nam. Now, in this episode, we have also confirmed that there is definitely something between Yoon Suk Chan and his mother that has yet to be revealed.
All For One decide to participate in the charity event. This leads Yoon Sul Chan’s group, to be removed from having to perform. Ma Joon Hee, the All For One leader convinces the teachers to allow All For One and Color Bars to battle against each other to decide who will perform in the charity event. Cha Do Nam is finally convinced to perform with the Yoon Sul Chan's group and reveals his beatboxing skills, while Jung Sun Woo decides to leave All For One to help out the other group.
Min Se Yi finds out that Jung Sun Woo is her childhood friend, and Kim Nana‘s feelings are revelaed. Min Se Yi overhears a portion of the conversation Yoon Sul Chan is having with Jung Sun Woo admitting his feelings for her, but misunderstands the subject of his attraction.
Kim Nana's powerful singing voice excites the group, making her the new lead singer, and finally, the battle between All For One and the Color Bars takes place.
The Color Bars lose to All For One, so Yoon Sul Chan uses his disappointment as an excuse to ask, Min Se Yi out on a date, but are put in an awkward situation by a reporter. Min Se Yi is upset when her mother suddenly comes back from New Zealand and Jang Sun Woo’s confesses his feelings. Radio and Cha Do Nam’s past is revealed.
Park Kyu Dong (Radio) is embarrassed when he is rescued from attempted suicide, and seeks out ‘Adam’ ahjussi . Yoon Sul Chan has to go to Japan to film an MV, but this provides an opportunity for Min Se Yi to decipher her feelings.
Embarrassed by the spontaneous kiss, Joon Sul Chan and Min Se Yi part confused. The next day, Min Se Yi spends the day thinking about Joon Sul Chan, who appears at school late. Nana opens up to Jung Sun Woo, and asks him to teach her how to play the bass. When Shim Eun Ha realizes JSC/MSY attraction for each other, she is heartbroken.
Jung Sun Woo can’t help feel saddened that his first love is interested in someone else. A producer from JDC scouts the school for a band battle, and sets up a rematch between the Color Bars and All For One, but Yoon Sul Chan has his group‘s comeback to worry about. ‘Adam’ ahjussi (Han Ji Woong)’s past is revealed, and Yoon Sul Chan confides in Min Se Yi about how his friendship with Jung Sun Woo became tarnished. Min Se Yi makes her decision between Yoon Sul Chan/Jung Sun Woo.
Despite being rejected, Jung Sun Woo is fired up to compete against Yoon Sul Chan for Min Se Yi, and once again, the Color Bars go against All For One for the variety show ‘Kiss of the Goddess’.
