From the introduction:
The Earth is threatened by a powerful creature who destroyed 70% of the Moon with its power, rendering into the shape of a crescent moon forever. The creature claims that within a year, Earth will also be destroyed by him, but he offers mankind a chance to avert this fate. In class 3-E at Kunugigaoka Junior High School, he starts working as a homeroom teacher where he teaches his students not only regular subjects, but the ways of assassination. The Japanese government promises a reward of ¥10 billion to whomever among the students succeeds in killing the teacher, whom they have named "Koro-sensei" (殺せんせー Korosensē, a pun on korosenai (殺せない or unkillable) and sensei (先生 or teacher)). However, this has proven to be an almost impossible task, as not only does he have several inhuman abilities at his disposal, including the capacity of moving at Mach 20, but he is also the best teacher they've ever had.