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 (i.e. 100 million USD) 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, unkillable) and sensei (先生, teacher)). However, this has proven to be an almost impossible task, as not only does he have several superpowers at his disposal, including the capacity of moving at Mach 20, but he is also the best teacher they’ve ever had, helping them not only to improve their grades, but also their individual skills and perspectives for the future.
As the series 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, and the students eventually learn the secrets involving him, the moon’s destruction and his ties with their previous homeroom teacher, including the true reason why he must be killed before the end of the year term.