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. Royal Knight Commander Tadaomi Karasuma tasks the students to kill Koro-sensei the Demon King, who in return will train them how to beat him as the "final boss". When Slime Kunudons attack the students, Nagisa Shiota bulks up his body and unleashes a small fireball to cause the Slime Kunudons to retreat.