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The story begins in a quiet middle school in Japan, where a soft-spoken teacher named Yuko Moriguchi stands before her students to announce her resignation. Her calm demeanor hides deep pain, and as the class fidgets and whispers, she begins to tell them why she is leaving. Yuko reveals that her four-year-old daughter, Manami, recently drowned in the school’s pool. The death had been ruled an accident, but Yuko tells the class that it was not accidental—it was murder. She slowly unveils that the killers are among them, two of her students, whom she refers to only as Student A and Student B. Her story becomes a chilling confession, one that changes every life in that classroom.
Student A, whose real name is Shuya Watanabe, is a brilliant but cold boy obsessed with scientific experiments and fame. He was raised by a mother who abandoned him, leaving him emotionally detached and desperate for recognition. He believed that killing Yuko’s daughter would bring him the kind of attention he craved. Together with Student B, Naoki Shimomura, a shy and weak-willed boy easily manipulated by others, Shuya devised a plan to electrocute the teacher’s daughter using a faulty circuit attached to her milk carton. The experiment went horribly wrong. When Manami collapsed, the boys panicked and left her to die in the pool. Yuko discovered the truth but realized that, because of their age, they would be protected by Japan’s juvenile justice system. So instead of seeking help from the authorities, she took justice into her own hands.
Yuko’s calm tone chills the students as she reveals her revenge. She had mixed HIV-positive blood from her ex-boyfriend into the milk cartons of Shuya and Naoki before her announcement that day. She tells them that the infection will take years to show but will ruin their futures. After the revelation, she quietly leaves the classroom, resigning from her job. The rest of the class sits in stunned silence, and from that moment, the two boys’ lives unravel.
Shuya, feeling no remorse, initially sees Yuko’s act as a challenge. He becomes even more consumed by his obsession with proving his superiority. He begins to record his thoughts and plans, believing himself to be a misunderstood genius. He recalls his childhood, where his mother’s neglect shaped his twisted desire to shock the world with acts of brilliance. He longs to create something extraordinary, even if it means taking lives. He starts crafting new experiments, tinkering with explosives, and studying the mechanisms of death. He feels that Yuko’s revenge has made him more famous in his own mind.
Naoki, on the other hand, collapses into madness. The quiet boy who followed Shuya’s lead becomes consumed by fear and guilt. His mother, Mrs. Shimomura, grows terrified as Naoki locks himself in his room, refusing to eat or speak. She tries to protect him, believing that the rumors about the murder are lies. But the boy’s paranoia grows. He believes that everyone knows his secret and that his life is already over. He begins to see blood everywhere and hallucinates that Yuko’s daughter is haunting him. The isolation drives him insane. His mother pleads with him to come out, but Naoki only sinks deeper into despair, becoming a prisoner of his guilt.
Meanwhile, Yuko tries to live a normal life after resigning. But she cannot let go of the past. She visits Manami’s empty room every night, her heart aching. Though she has taken her revenge, it brings her no peace. She follows the news of her former students, aware that their lives are disintegrating. Naoki’s mother begins appearing on television, speaking about the dangers of false accusations against children. She insists that her son is innocent and that society has destroyed him. But Yuko knows the truth. Her revenge is working, but she wonders if she has become as cruel as the killers themselves.
Shuya’s obsession with proving himself takes a new direction. He finds a new accomplice, a girl named Mizuki Kitahara, a quiet and lonely classmate drawn to his intelligence. She feels invisible at school and sees Shuya as someone who understands her pain. But Shuya only uses her. He confides in her about his plan to build a bomb powerful enough to blow up the entire school. He tells her that the world needs to recognize his genius. Mizuki, disturbed yet fascinated, follows him as he tests explosives near abandoned areas. She believes she can save him, that her love might change him. But when she confronts him about his lack of empathy, Shuya coldly tells her that she is just another test subject. Mizuki realizes she has fallen in love with a monster. In despair, she follows him to a secluded park where he plans to test his latest bomb. When she tries to stop him, he mocks her. Overwhelmed by pain, Mizuki slits her own wrists, dying in front of him. For the first time, Shuya feels something close to guilt—but it quickly turns into rage. He blames Yuko for everything. He decides his final masterpiece will be an explosion that destroys his school and everyone in it.
As Naoki’s mental condition worsens, his mother becomes desperate to cure him. She locks herself inside the house with him, cutting off contact with the outside world. The stench of decay fills their home. She tries to convince him that nothing bad happened, that it was all a misunderstanding. But Naoki screams at her, accusing her of lying. He begins to see his mother as an enemy. One night, during a violent outburst, he kills her. Covered in blood, he sits motionless in the room, finally consumed by madness. His mother’s death completes his descent into darkness—the final result of Yuko’s vengeance.
Meanwhile, Yuko learns about Shuya’s new plan. Through media reports and school gossip, she discovers that Shuya has built a powerful bomb and intends to detonate it at the school’s graduation ceremony. Realizing that he intends to take innocent lives, she decides to intervene. She tracks him down, following his movements through clues left in his online postings. When she finds him, he is standing near the school grounds, carrying the bomb in a backpack, smiling with twisted satisfaction. He is waiting for the perfect moment to press the trigger and achieve the fame he’s always desired.
Yuko confronts him calmly. She tells him she understands his pain, that his need for attention stems from abandonment, but that his life has become nothing but hatred. Shuya mocks her, calling her a murderer for what she did to him and Naoki. He says he doesn’t care anymore—he just wants to see the explosion, to feel the world acknowledge his existence. But Yuko’s tone never wavers. She tells him that she has already moved his bomb. While Shuya was preparing for his final act, she had discovered and relocated the device. The explosion he planned will not destroy the school—it will destroy something far more personal. Shuya’s arrogant smile fades as Yuko whispers that she has placed the bomb in a location that will make him truly understand loss. The timer ticks down, and realization dawns on his face. The bomb is now at his mother’s hospital, where she had recently been found alive after years of estrangement. Before he can react, the explosion echoes through the distance.
Yuko looks at him with cold eyes and says, “Just kidding.” The story ends in silence, leaving it uncertain whether she told the truth or if the explosion was only in Shuya’s mind. The weight of her vengeance, the madness of the students, and the collapse of innocence linger like an echo.
The tale unfolds through the intersecting confessions of the teacher, the students, and those around them—each voice revealing how guilt, vengeance, and broken love destroy lives. In the end, no one is innocent. Yuko’s vengeance consumes her soul as deeply as it consumes the killers. Shuya’s brilliance turns into a monstrous hunger for attention. Naoki’s guilt devours his sanity. Every confession uncovers another wound, another secret, until the entire tragedy becomes a reflection of how the line between justice and cruelty can vanish when a heart breaks beyond repair.