Korosensei was the homeroom teacher for Class 3-E of Kunugigaoka Junior High School, and the central character and main antihero of Assassination Classroom. He claimed to be responsible for creating the permanent crescent moon and said that he planned to destroy the earth after “teaching” Class 3-E for a year.
Earth is threatened by an enormously powerful monster who destroyed 70% of the Moon with its power, permanently rending it into the shape of a crescent. The monster 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 whoever among the students succeeds in killing the monster, 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 entire 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