A strange new teacher is introduced to Class 3-E at Kunugigaoka Junior High School. The teacher is a round-headed, tentacled octopoid creature capable of moving at speeds of up to Mach 20 and whose color changes depending on his mood. Prior to introducing himself to his new class, he managed to carve out a large chunk of the moon to become a permanent crescent moon. In a year, he plots to destroy the earth, but decides to teach a junior high school class in the meantime for an unknown reason. With this strange creature having effectively taken over the world, the government reluctantly complies with his request on the condition that he does not harm his students.
Affectionately, the students of Class E call him Korosensei, which is pun on the term korosenai (殺せない lit. unkillable or can't be killed) and sensei (先生). These same students greet him everyday with assassination attempts in their journey to save the world by eventually murdering their own teacher, and to earn 10 billion yen from the government.