Фильм представляет собой встречу Кармы и Нагисы в месте проведения занятий класса убийц спустя 7 лет после произошедших событий второго сезона. В дружеском разговоре герои решают поностальгировать и вспомнить всё произошедшее за тот год учёбы в средней школе Кунугигаока. Экранизация является кратким пересказом двух сезонов с акцентированием внимания на событиях со стороны вышеуказанных героев.

A year can change a person's life forever. The 365 days Class 3-E of Kunugigaoka Junior High spent with their eccentric teacher, Koro-sensei, certainly did. Carrying the memories of that year close to their hearts, alumni Nagisa Shiota and Karma Akabane return to their former classroom to recall the events of that momentous time of their lives.

Nagisa and Karma are reminded by the familiar rooms, desks, chalkboard, and the class album 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—and the time spent in the bizarre yet exceptional "Assassination Classroom."

[Written by MAL Rewrite]