Sakuranomi is diagnosed with HIV and breaks off his engagement with Moriguchi, who is already pregnant with their daughter.

Manami is found drowned in the school pool and her death is officially ruled accidental by authorities.

Moriguchi investigates Manami's death and determines that two of her students, Shūya and Naoki, are the killers.

Moriguchi secretly injects Sakuranomi's HIV-positive blood into the milk cartons assigned to Shūya and Naoki.

Moriguchi delivers her retirement speech, publicly revealing the murder and identifying Shūya and Naoki to their classmates.

After Moriguchi's speech, Naoki stops coming to school and locks himself inside his bedroom.

Shūya becomes the target of relentless, school-wide bullying after classmates assume he has contracted HIV.

Shūya and Mizuki meet after school and together discover the milk cartons had contained no blood at all.

Shūya ends the classroom bullying by threatening the lead bully with his blood and a forced mouth kiss.

Mrs. Shitamura secretly drugs Naoki with sleeping pills and cuts his hair while he sleeps.

Naoki confesses to his mother that he knew Manami was still alive when he dropped her in the pool.

Mrs. Shitamura carries out a murder-suicide plan; she dies from a stab wound while Naoki survives.

Mizuki tells Shūya his mother truly abandoned him; Shūya reacts by strangling and killing her.

Shūya remotely detonates his bomb but no explosion occurs at the school; his phone immediately rings.

Moriguchi reveals she moved Shūya's bomb to his mother's university laboratory, where it detonated and killed her.

Moriguchi, a middle school science teacher, loses her four-year-old daughter Manami in what is ruled an accidental drowning, but she uncovers that two of her students, Shūya and Naoki, are responsible. Unwilling to trust Japan's lenient juvenile justice system, she exacts revenge by placing HIV-positive blood in the killers' milk cartons during her retirement speech, publicly branding them to the class. In the new school year, Naoki retreats into isolation while Shūya endures brutal bullying from classmates who believe he has contracted HIV, though Shūya and Mizuki eventually discover the milk was never truly contaminated. As both boys spiral into psychological crisis, each ultimately kills his own mother — Naoki during a violent emotional breakdown, and Shūya unknowingly through a remotely detonated bomb. Moriguchi, who has been engineering events from the shadows since her retirement, reveals during a final phone call that she moved the bomb to Shūya's mother's laboratory, ensuring his detonation killed her. Only then does Moriguchi consider her revenge for Manami's death complete.

The core events of Manami's murder and Moriguchi's carefully orchestrated revenge.

Moriguchi becomes a single mother after Sakuranomi ends their engagement following his HIV diagnosis.

Manami secretly slips away during school hours to visit the pool and feed a stray dog living nearby.

Shūya builds a high-voltage shocking coin purse and uses Moriguchi as an unwitting test subject.

Shūya places third nationally at a science fair, but the Lunacy Incident child murder case eclipses his media coverage.

Shūya concludes that juvenile crime earns more media attention than scientific achievement and decides to commit murder.

Naoki is assigned to clean the school pool as a punishment and discovers Manami regularly feeding the dog there.

Naoki suggests Manami as a murder target in order to impress Shūya and secure a place in his friendship.

Shūya and Naoki give Manami the Snuggly Bunny pouch containing the Execution Machine, electrocuting her into unconsciousness.

After Manami is electrocuted and presumed dead, Shūya rejects Naoki's friendship and abandons the pool area alone.

Werther replaces Moriguchi as homeroom teacher and announces he will not read her notes about the students.

The class creates a bullying point system that forces every student to torment Shūya or become a target themselves.

The class retaliates against Mizuki by forcing her to throw milk at Shūya and kiss him publicly on the mouth.

Werther conducts weekly home visits to Naoki's house with Mizuki in tow; Mrs. Shitamura grows increasingly disillusioned.

At a fast food restaurant, a young girl spilling milk on Naoki triggers a hallucination and causes a public breakdown.

Shūya visits his mother's university and discovers she has remarried the science fair judge and is pregnant.

Before the murder, Shūya's obsession with capturing his absent mother's attention through science leads him from a nationally ranked science fair to a calculated plan to commit a sensational crime, enlisting the eager but underestimated Naoki as an accomplice. Together they lure Manami to her death using a rigged Snuggly Bunny pouch, but Shūya's rejection of Naoki's friendship immediately afterward sets both boys on a collision course with disaster. When the new school year begins, the replacement teacher Werther refuses to read Moriguchi's notes about the class, leaving himself unprepared for the volatile classroom environment he inherits. The entire class is swept into a violent bullying campaign against Shūya, with even reluctant students like Mizuki coerced into participating under threat of becoming targets themselves. Werther's weekly home visits to the shut-in Naoki prove hollow, as he has no real plan to help and only grows more patronizing. When Shūya finally travels to his mother's university, the sight of her new husband and pregnancy destroys his last remaining hope for reunion.

The backstory behind the murder plot and the escalating chaos of the new school year.

Manami throws a public tantrum over a Snuggly Bunny pouch at a mall store; Moriguchi refuses to buy it.

Naoki calls Moriguchi for help after being taken to a police station; the tennis coach arrives in her place.

Shūya invites Naoki to his secret laboratory, shows him the Execution Machine, and recruits him as an accomplice.

After Shūya leaves the pool believing Manami is dead, Naoki discovers she is still alive and drowns her himself.

After the bullying ends, Shūya and Mizuki develop a friendship that eventually becomes a romantic relationship.

Werther delivers a class card to Naoki's house whose first letters spell out the phrase "DIE, MURDERER."

Werther shouts his final motivational speech at Naoki's bedroom window, mentioning that Shūya has been attending school.

Mizuki is unable to deliver her letter to Naoki explaining he does not have HIV.

Naoki obsessively washes his own dishes and clothes while refusing to bathe or accept physical touch from anyone.

Mrs. Shitamura takes Naoki to see a psychologist in an attempt to address his increasingly erratic behavior.

After his mother's forced haircut, Naoki shaves his head, bathes for the first time in months, and smears blood on store products.

Shūya's mother leaves him behind after her divorce and takes a prestigious university engineering job, effectively abandoning him.

Mizuki grows increasingly paranoid that she may be the Lunacy killer because she possesses similar chemicals.

Sakuranomi discovers Moriguchi's plan before the retirement speech and secretly replaces the contaminated milk cartons with clean ones.

Moriguchi withholds Shūya's mother's final words from him during their phone call as her last act of revenge.

The turning points that define the story begin with small, seemingly ordinary moments — a refused toy, a missed phone call, a friendship brutally rejected — each of which ripples outward into catastrophe. Naoki's private discovery that Manami was still alive after Shūya left the pool transforms him from a passive accomplice into her actual killer, a truth he buries until it unravels him entirely. During the chaotic new school year, Mizuki quietly uncovers that the milk was never contaminated, a secret she weaponizes to help end the bullying, while Naoki's mental deterioration culminates in a bizarre public act of self-destruction. Shūya's backstory, revealed late in the novel, exposes how his mother's abandonment warped his every ambition, and Mizuki's clear-eyed articulation of that truth costs her life. In the final chapter, the full architecture of Moriguchi's revenge is laid bare: Sakuranomi had already neutralized her original milk plot, and Moriguchi had been orchestrating events through Werther all along. Her last and most personal act of vengeance is to deny Shūya the one thing he has wanted most — his mother's final words.

The hidden decisions, quiet discoveries, and shocking revelations that shaped every character's fate.