How Not to Summon a Demon Lord

Diablo, Shera, and Rem head out to the plains to test out new magic. While using said Flight Magic, Diablo spots the light of magic energy rising up from deep within the forest, so he makes haste towards its source. However, his spell gives out half way, and he goes plummeting down! When he lands, he finds the Head Priest, Lumachina under attack from the Paladin, Gewalt.

Diablo and the others leave Faltra and arrive at Zircon Tower. There they meet an adventurer, Horn, who talks to Diablo about a new dungeon that has appeared — a dungeon that Diablo suspects is the one he created. Meanwhile, Lumachina gets into an argument with a man claiming to belong to the Paladin Brigade who kicked aside a woman carrying her baby…

Lumachina realizes that Battuta is conducting a large scale curse, threatening the lives of others, but he puts her and Rem to sleep. The two later awaken in an underground space where believers are conducting the ritual. After Horn tells them of Lumachina and Rem’s capture, Diablo and Shera take action to rescue them!

Diablo and the others were heading to the dungeon to obtain the hidden treasure that can cure the Death Knell Disease when Laminitus and her army surrounds them and attacks. Their party manages to reach the entrance to the dungeon, but there are now six marks on Lumachina’s body, and it’s said the victims of the Death Knell Disease will die once there are nine marks…

After rescuing Horn, Diablo learns of Horn’s secret. Meanwhile, Shera, Rem, and Lumachina make progress through the dungeon and reach the twelfth underground floor. When Rem opens the door, they find the man pursuing Lumachina — Gewalt. To make things worse, a Large Black Dragon appears with a terrifying blast of wind!

Diablo resolves to return to Zircon Tower at once after learning the city was under attack by the Demon Lord Army. Meanwhile the desert becomes a battlefield as Laminitus and her soldiers wage battle against the Demon Lord Army's Commander, Varakness. Laminitus struggles against the overwhelming might of the Fallen, and just as the final blow is about to be dealt to her, Diablo appears with a flash.

Lumachina asks Diablo to accompany her to the royal capital, so he decides to stop by Faltra City on the way. Diablo's group is worried about Klem and Edelgard after such a long time apart, and Sylvie informs them that they're at a bakery. Apparently the Fallen, Edelgard, had started a part-time job. Curious, Diablo and the others go to investigate...

Diablo and his group arrive at the royal capital, Seven Wall, but the guards stop them at the gates. That explosive situation is resolved by the imperial knight, Alicia. She was awaiting their arrival and leads the group to an inn. That night Diablo reveals to Alicia that the church is out to kill Lumachina...

Diablo and the others are locked up in the sanctuary by Vishos and kept apart from Lumachina. Meanwhile, Horn managed to escape, but she's torn between her desire to flee all by herself and her desire to help her friends. That's when Babalon, the Level Up Goddess that resides within the Holy Grail, appears before Horn.