The film opens with a dramatic calamity as a Japanese military unit tasked with capturing the being blamed for destroying more than 70% of the Moon is mercilessly attacked by a mysterious tentacled entity. The events quickly set the tone for a story that blends science fiction, school life, and high-stakes danger.
Nagisa Shiota [Ryosuke Yamada]—a ninth-grade student at Kunugigaoka Junior High—narrates how he ends up in the infamous Class 3-E, a group shunned by the school and used by the government as a live target in a bid to eliminate Koro-sensei [Kazunari Ninomiya], a tentacled mutant blamed for the cosmic catastrophe. The class is given a twin purpose: to study and to assassinate their mysterious tutor before graduation, all while the world holds its breath to see if his doom might truly come at the end of the school term.
In the classroom, Koro-sensei proves to be an extraordinary teacher who forms genuine bonds with his students, despite being their ultimate target. He imparts lessons that are as practical as they are perilous, and his unorthodox methods push the students to grow in unexpected ways. He is assisted by government-affiliated mentors, Tadaomi Karasuma [Kippei Shiina] and Irina Jelavić [Kang Ji-young], whose presence adds pressure and strategy to the students’ mission and daily life.
Throughout the term, Class 3-E welcomes three new faces: Karma Akabane [Masaki Suda], whose recent suspension led him to join the class; Ritsu, an AI-box taking the form of a girl; and Itona Horibe [Seishirou Katou], a self-proclaimed “blood brother” of Koro-sensei who arrives under cloaked guardianship after a clash with his rival. The balance of power inside the school shifts when Akira Takaoka [Masanobu Takashima], a drill-instructor-type rival, briefly replaces Karasuma as the P.E. teacher, challenging the class to prove themselves by directing an attack on him with a well-timed knife strike delivered by Nagisa.
As finals approach, the five academic subjects—Chemistry, English, Home Economics, Japanese, and Maths—offer a tantalizing incentive: top scorers will earn the right to sever Koro-sensei’s rubbery tentacles, a move that would slow him considerably. Six students manage to reach the top, including multiple first-place finishes in Home Economics, setting the stage for a high-stakes plan to kill Koro-sensei during the summer break. The students reflect on Aguri Yukimura [Mirei Kiritani], a teacher from their past who left a lasting imprint on them as they chase the ambitious goal.
The tension crescendos when the government seizes the chance to execute Koro-sensei publicly, motivating the class to fight for him. Yet in a tense turn, Koro-sensei contorts into a ball to dodge the assault, and Takaoka poisons several students to force a bargain. He demands Koro-sensei in exchange for antidotes, throwing Kaede Kayano [Maika Yamamoto] into danger as she is kidnapped unless the tentacled teacher is handed over. Nagisa and Karma join forces to neutralize Itona Horibe, luring him into open rain and cutting his tentacles, while Ritsu alerts the team to Takaoka’s kidnapping and the looming ultimatum.
With cunning and teamwork, Class 3-E turns the tables and defeats Takaoka without surrendering Koro-sensei. Yet the government’s plan to end him in public fails when Koro-sensei is captured only to survive the ceremony and continue teaching Class 3-E for the next term, with Itona Horibe joining as a permanent pupil. In the quiet aftermath, Koro-sensei remembers the promise he made to someone from his past: Aguri Yukimura [Mirei Kiritani], whose memory frames his choices and his resolve to protect his students despite the danger he invites.
