The film opens with the Japanese military's failed attempt to capture whoever it is that has destroyed half of the Moon, which sees the death of the entire squad sent to capture it. Meanwhile, Nagisa Shiota, a ninth-grade student of Kunugigaoka Junior High School, narrates of how he came to be sent to the situation he is in: he is included in Class 3-E, which is reserved for delinquents and other undesirables of the school and is a laughing stock among the school. However, Class 3-E is actually a cover up for the Japanese government in their attempt to assassinate Koro-sensei, a tentacled creature apparently responsible for the Moon's destruction. The students have to assassinate him before graduation using either a special knife or BB pellet, the time when he plans to destroy Earth, but at the same time also have to study both academic and assassination disciples from him. In teaching, Koro-sensei is assisted by the government-affiliated Tadaomi Karasuma and Irina Jelavić, a Serb assassin. Despite being fully aware that he is their ultimate target, Koro-sensei is able to bond with Class 3-E, who in turn regard him as their indispensable teacher. When asked why he took the job, he says it is done to fulfill a promise made to someone. Throughout the school term, Class 3-E welcomes three additional students: Karma Akabane, who has recently come out of his suspension due to protecting a former Class 3-E pupil from being bullied by Class 3-A students; "Ritsu", an AI-box displaying the form of a girl; and Itona Horibe, the also-tentacled self-proclaimed "blood brother" of Koro-sensei who is carried away from the school by his cloaked guardian after losing a match against the latter. At one time, Karasuma's position as P.E. teacher is briefly replaced by his rival, Akira Takaoka, a drill instructor-esque individual. He is challenged by Karasuma, who successfully sends Nagisa to threaten him, thereby expelling him from the school. Before the finals, Koro-sensei promises that the top scorers of the five academic subjects will be awarded chance to cut his tentacles, which will greatly slow him. When the results for the finals are out, three students are able to jointly top the Home Economics test, thus granting seven students the chance to cut the tentacles. The students plan the killing during the summer holiday, where they reminisce about their kind past teacher, Aguri Yukimura. Eventually, Class 3-E manages to bind Koro-sensei thanks to Nagisa's observation of his weakness: water. However, Koro-sensei transforms into a ball to avoid being killed. Suddenly, the class is interrupted by a vengeful Takaoka, who poisons several students. He agrees to trade the antidotes for Koro-sensei. Nagisa and Karma team up to incapacitate Itona, now working for Takaoka, by luring him into open rain and subsequently cutting his tentacles. However, they are alerted by Ritsu that Takaoka has kidnapped Kaede Kayano and threatens to kill her unless Koro-sensei is handed over. With a trick, Class 3-E turns the table over and defeats Takaoka without giving in. Seeing that Koro-sensei is vulnerable, the government takes him and proceeds to do a public execution in front of his grieving students. However, he manages to survive the attempted killing and is allowed to continue teaching Class 3-E for the next term, with Itona now enrolling permanently. Koro-sensei then reminisces about the person he made his promise with: Aguri Yukimura.
It opened in 313 screens across Japan and $3.42 million on 351,000 admissions in its first weekend. It ultimately grossed over $20 million in Japan alone. Film School Rejects awarded it a score of B+, saying "Assassination Classroom is an absolute blast that not only survives its ridiculous setup but makes something truly special with it." We Got This Covered awarded it three and a half out of five, saying "Assassination Classroom is heartwarming J-Pop mayhem that tells of an alien schoolteacher who plans to destroy humanity, and the class of misfits who must kill him before graduation. Just soak that in." Twitch Film said "This was the lighthearted and feel good Japanese film I was waiting for all festival long." Otaku USA was more negative, saying "as a piece of storytelling, Assassination Classroom felt more like a series of non-cohesive chapters than a fleshed-out whole, probably thanks to its manga origins."
