Nestled in the mountains near the prestigious Kunugigaoka Middle School stands a small, dilapidated building that serves as the home for the delinquents and dropouts of Class 3-E. Disdained by their peers, the students in this class seem to have little hope of advancing their academic careers. That changes, however, when the national government assigns them the task of eliminating the greatest threat to their planet: their new teacher. Having already destroyed the moon, the octopus-like professor—nicknamed "Koro-sensei"—has now threatened to destroy the Earth by March of the following year. In light of their mission, the students discover that killing him is easier said than done. Not only can Koro-sensei move at speeds of up to Mach 20, but he can also withstand almost every earthly weapon. Ironically, he also proves to be one of the best teachers Class 3-E has ever had. Training the class to excel in both their studies as students and their skills as assassins, Koro-sensei is confident that his students' ingenuity and indomitable will could return them to the main campus. Through trial and error, Nagisa Shiota, along with the other students of Class 3-E, must figure out Koro-sensei's weaknesses—and quickly, for the very fate of the world depends upon it.