The story takes place in Class E of the third grade of Kunugigaoka Junior High School. On the surface, this is an ordinary class, but in fact, it is full of highly skilled and ruthless killers. Every year, they receive assassination missions from all over the world through special channels to "solve problems" for the heads of various countries. This time, they have a unified goal-to assassinate the head teacher, Mr. Koro (voiced by Jun Fukuyama). Mr. Koro is a magical existence. His appearance is not human, but he has extremely sensitive nerves and extremely quick reaction. No one knows where he came from, and no one knows why he became the head teacher of Class E of the third grade. Because he wants to destroy the earth, Mr. Koro has become a high-priced bounty on the assassination list. As a student of Class E of the third grade, as long as he successfully eliminates Mr. Koro, he can graduate. In this way, the daily life full of dangers around this mysterious teacher unfolds.