Junior high school teacher Yuko Moriguchi (Takako Matsu) announced to her class that she would resign before spring break. Moriguchi revealed that because the HIV-positive biological father of her daughter Manami was ill, she used to bring Manami (Mana Ashida) to school with her. One day, after school, she returned to the room where Manami was but found her gone. Her daughter was later found drowned in a school swimming pool. She then went on to explain that two pupils in her class, whom she dubbed "Student A" and "Student B", had murdered her four-year-old daughter.
She found a small bunny purse amongst Manami's belongings which should not have belonged there, which led her to question Shuya, one of her students, who immediately admitted killing Manami, and mocked her compassionate reaction to his feinted expression of remorse of "just kidding".
Having revealed their identities, she explained that, because they, as minors, are protected by the Juvenile Law of 1947, turning them in wouldn't make a difference. So to inact revenge on them, she admits injecting her late daughter's biological father's HIV-contaminated blood in the milk cartons of the two students who she claimed to have murdered her daughter. The rest of the film switches between the aftermath of Moriguchi's confession and the events before the confession through a series of first-person narratives from Moriguchi and her three students. Naoki Shimomura (Kaoru Fujiwara), Student B, became a shut-in because he believes he had acquired AIDS from drinking HIV-contaminated milk. His mother (Yoshino Kimura), after spotting the hidden malicious messages in well-wishes cards from Naoki's classmates, realized her son was somehow involved with an earlier incident. She decided to commit murder-suicide to free her son and herself from torment, but in the ensuing struggle, Naoki kills her. The police arrested him for the murder. Shuya Watanabe (Yukito Nishii), Student A, explained that his mother abused him before leaving to pursue her scientific ambitions. He confessed that her abandonment had driven him to thrive in Science, from making small inventions to recording his killing and dissecting animals. His first public invention, an electric anti-mugger wallet, earned him a science fair award, but it failed to make the headlines, as the media was busy covering "Lunacy Incident".
Shuya and Naoki's recollections revealed that Shuya said he had upgraded the anti-mugger wallet, and decided to try it out on someone, and roped Naoki in to help with his plan. They decided on Moriguchi's daughter Manami. However, when they tested it on Manami, Shuya's device only managed to render Manami unconscious, which Shuya mistook as an instant death, who then told Naoki to tell the world that Shuya did it. Enraged Naoki then threw the conscious Manami into the pool where she drowned, therefore proving he was the more effective killer.
The classmates forced classmate Mizuki Kitahara (Ai Hashimoto) to kiss Shuya as part of their bullying against Shuya. Mizuki later told Shuya she believed Moriguchi had lied about the blood-contaminated milk as it was an implausible method of transmission. After spending time together, Mizuki eventually confesses to Shuya that she identified the "Lunacy Murder" girl, who poisoned her parents, as her other self. They soon became romantically involved, but Shuya kills her after a confrontation over his Oedipus Complex, and claiming that she was nothing but "a means to relieve boredom".
After finally finding out messages left with his mother's name and work place, Shuya visited the university where his mother worked, expecting to reunite with her, but discovered she had remarried. Believing she had forgotten him, he planted a bomb in his school's sport hall where the graduation ceremony would be held and he was to give a speech. However, to his surprise, the bomb did not go off during the ceremony. As he tried to work out what had happened, he received a call from Moriguchi, who said that she had relocated the bomb to his mother's office she explained that it was her ultimate revenge, to let his mother die of his own hands, but his redemption would now begin. As the screen darkens, Moriguchi chuckles and says, "Just kidding."