It has been six months since the start of senior year, and yet the Class 3-E of Kunugigaoka High School has not been able to assassinate their tentacled teacher, Koro-sensei, who will blow up the Earth on graduation day. Due to the school festival to be held, assistant teacher Tadaomi Karasuma instructs the class to give all they got, since Koro-sensei will be distracted, making him an easy target of killing. Karasuma also insists that the class should assassinate Koro-sensei before the government-sponsored assassin Red Eye do the same. However, neither the students nor Red Eye are ultimately able to do the task, and Koro-sensei even convinces Red Eye to give up his job for the sake of his students.