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.

Karasuma visits Pope Gakuhō Asano, who devises a plan to lure Koro-sensei into a trap. Koro-sensei and the students of Class 3-E go into the Ruins of Tribulation with three chambers hosted by a Guardian Kunudon.

The students have a new addition to their class, her being the Autonomous Intelligence Magic Stone Tablet, who can perform various forms of sorcery.

Itona Horibe the Silver Berserker crashes through the classroom, and his guardian Mage Shiro tells Koro-sensei that Itona will be the new Demon King.

Class 3-E finds a treasure chest deep in a cave, but it turned out to be empty when it is opened. To make matters worse, everyone switches bodies with someone else.

Kaede Kayano witnesses Nagisa and her pudding being abducted by Akira Takaoka the Merciless, ringleader of the Taka Gang. The students of Class 3-E goes to Fukuma Tower, where Nagisa is being held captive. On the second floor, Yuzuki Fuwa takes down Smog the Noxious by breaking the fourth wall.

The female students go to Irina to brew a potion for sacks of chocolates. On Valentine's Day, some of the female students give some of the male students the sacks of chocolates, but it turns the male students into monsters for a day.

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.