Assassination Classroom

Earth is left in jeopardy after an enormously powerful tentacled creature suddenly appears and destroys 70% of the Moon, leaving it in the shape of a crescent. The creature claims that within a year, he will destroy the planet next. However, he offers mankind a chance to avert this fate.

In class 3-E, the End Class of Kunugigaoka Junior High School, the creature starts working as a homeroom teacher where he teaches his students regular subjects, as well as the ways of assassination. The Japanese government promises a reward of ¥10 billion to whoever among the students succeeds in killing the creature, whom they have named "Koro-sensei". However, this proves to be a virtually impossible task, as not only does he have several superpowers at his disposal including accelerated regeneration, visual cloning, an invincible form, and the ability to move and fly at Mach 20, but he is also the best teacher they could ask for, helping them to improve their grades, individual skills, and prospects for the future.

As time goes on, the situation gets even more complicated as other assassins come after Koro-sensei's life, some coveting the reward, others for personal reasons. The students eventually learn the secrets involving him, the Moon's destruction, his ties with their previous homeroom teacher, and the true reason why he must be killed before the end of the school year.