Nagisa Shiota and the rest of Kunugigaoka Junior High's Class 3-E have fifteen days left on their deadline to assassinate their tentacled teacher, Koro-sensei. The film goes back over the year behind them with side stories the TV series never had room for: the class's art projects, a swimming unit, dedicated assassination drills, and a Valentine's Day subplot, plus glimpses of what Koro-sensei does after class lets out. Rather than advancing the main plot, it fills gaps in Class 3-E's last year together.