When her husband became more ill, junior high school teacher Yuko Moriguchi began bringing their daughter Manami to school with her. One day, Manami was found dead in the school's swimming pool. Some time later, Yuko calmly announces to her undisciplined and disrespectful students that she will resign before spring break, and reveals that two students in the class (whom she dubs "Student A" and "Student B") murdered Manami. Yuko figures out who killed Manami by finding a small purse with a bunny mascot on it among Manami's belongings, leading her to question Naoki Shimomura (Student B). A flashback shows Yuko in a shop, where she tells Manami she cannot buy the purse as she has just bought a jacket featuring the same bunny mascot; Shuya Watanabe (Student A) sees her and loudly questions why Manami cannot have the purse, irking Yuko. Shuya later visits Yuko in the school's empty science lab and admits to killing Manami before rushing to jump out of a window, pretending that he is about to kill himself out of regret, but he then mocks Yuko's plea with him not to jump by saying "just kidding". Yuko tells her class that she will not name the students who killed Manami for the same reason that she did not tell the police: the students are minors and thus unable to be arrested due to the
Juvenile Law of 1947, a fact which also allows Shuya to continue taunting her about the murder. However, she tells the class how she figured out who committed the murder, and purposely makes it extremely easy for the other students to deduct that she is referring to Shuya and Naoki. She believes that her role as a teacher makes her responsible for teaching the boys a lesson by making them atone for their actions, and thus reveals that she injected their milk cartons with the blood of Manami's father. The other students react with panic and immediately move away from the boys. The rest of the film switches between the aftermath of Yuko's confession and the events before the confession via narratives from Yuko, Shuya, Naoki, and the boys' classmate Mizuki Kitahara. Naoki
becomes a because he believes he has contracted HIV from the contaminated milk. His mother realizes he was involved in Manami's death and plans to commit to free them both from the shame, but he stabs her to death in the ensuing struggle and the police arrest him. It is revealed that he did not throw Manami's corpse into the pool to make her murder look accidental as previously claimed, but knew she was simply unconscious after Shuya's botched murder attempt and threw her in anyway. Shuya later explains that his mother abused him before abandoning the family to pursue a career in science, and her abandonment made him obsessed with scientific accomplishments and experiments, ranging from creating small inventions to documenting his murder and dissection of various animals. His first public invention, an electrified wallet, earned him a
science fair award and made the local news; however, he was kept out of the headlines by a murder case in which a young girl poisoned her parents. Shuya upgraded the wallet, decided to try it out on someone, and roped in Naoki to help. They decided to test the device, which had since been installed in the bunny purse, on Manami. However, it rendered her unconscious by the pool; Shuya mistook this as having killed Manami and walked away, proud of his achievement. Naoki then threw Manami into the pool, where she drowned, making him the real killer. Yuko comments on the irony of the situation, with Shuya failing to kill someone despite wanting to and Naoki killing someone despite claiming that he did not want to. Mizuki tells Shuya that she believes Yuko lied about the contaminated milk, as it was an implausible method of HIV transmission. She confesses to him that she identifies with the girl in the poisoning case, and the two become romantically involved, but he kills her and stores pieces of her body in his fridge after a confrontation over the
abandonment complex caused by his mother. Shuya receives an email from his mother, who says she wants to see him. He travels to the university where she works, expecting to reunite with her, only to discover that she has remarried and is away on her honeymoon. Believing she has forgotten him, he plants a bomb in his school during graduation and links it to his phone, aiming to kill himself and everyone else in the school during his speech. To his surprise, the bomb does not go off when he hits the button on his phone. He then receives a call from Yuko, who reveals that she faked the email from his mother and figured out his plan by watching the video manifesto he uploaded to his website. She subsequently relocated the bomb to the office of his mother, who just returned from her honeymoon. Yuko explains that this is her ultimate revenge: causing Shuya to kill his own mother. Shuya has a breakdown in the school assembly hall as Yuko arrives, telling him that his path to redemption can begin now that she has finally had her revenge, but she then laughs and says "just kidding". ==Cast==