In the blink of an eye, the night sky is forever changed when, with a great explosion, the moon is blasted into a permanent crescent. Confronting the governments of the world, a yellow tentacled monster boasts that it was the one responsible for the catastrophe, and in a year's time, it will do the same to Earth. Zooming around at Mach 20, the creature makes a peculiar demand: a teaching position at the prestigious Kunugigaoka Junior High School. Desperate to keep the creature in one place, the government agrees, and so the students of Class 3-E find themselves faced with an impossible task—assassinate the creature responsible for destroying the moon. But as the enigmatic creature—whom they dub Koro-sensei—educates them in both assassination and academia, the children slowly find the courage to rise up and take aim at their target: the greatest teacher the world has ever seen.