A year can drastically alter a person's life. Class 3-E at Kunugigaoka Junior High spent 365 days with their eccentric teacher, Koro-sensei. Alumni Nagisa Shiota and Karma Akabane return to their former classroom to recall the events of that pivotal year in their lives, carrying the memories of that year close to their hearts. The familiar rooms, desks, chalkboard, and class album remind Nagisa and Karma of the events that shaped them into "assassins" and prepared them for the real world. That is the legacy of their octopus-like teacher, who had introduced himself by threatening to destroy the world if they didn't kill him by the end of the school year, as well as the time spent in the bizarre yet exceptional "Assassination" class.