Set in a role-playing game world, the students of Kunigigaoka Junior High School's Class 3-E are heroes-in-training filled with various bugs.

Koro-sensei and the students of Class 3-E travel to the northern cave to confront Karma Akabane the Red Devil, who is known to be unlucky when he is cocky, especially when washtubs randomly land on his head.

Koro-sensei and the students of Class 3-E go to a nearby village, where the village chief warns them of Irina Jelavić the Seductive Witch.

When Koro-sensei was a human, he was thought of as a hero, but became bored of his work. He was assigned by Aguri Yukimura to be a Demon King and live in a castle, since his predecessor (actually Neuro from Neuro: Supernatural Detective) retired from his position.

The students of Class 3-E storm through the castle to get to the Demon King. Many students sacrifice themselves for each other in many traps as they travel to the throne room.