Class 3-E at Kunugigaoka Junior High welcomes a highly unusual teacher: an immensely powerful creature capable of moving at incredible speed and responsible for destroying a large part of the Moon. He announces that he will destroy Earth in one year unless his students manage to assassinate him before then. Nicknamed Koro-sensei, he nevertheless proves to be a caring and exceptionally effective teacher, helping his students improve while avoiding every attempt on his life. Between classes, training sessions, and assassination missions, the students gradually come to understand their strange teacher while learning to overcome the failures that caused them to be rejected by the rest of the school.