Juzo Murasaki is a quiet young man who takes a job at a construction site, only to discover that his new boss is Toru Akai, the relentless bully who terrorized him throughout middle school. To make matters worse, Juzo has just moved into a small apartment, and Akai lives with his wife Nozomi and their toddler son in the unit directly above him. While Akai does not recognize his former victim, he continues his abusive behavior, casually humiliating Juzo and other workers on the job. The stress of this confrontation triggers something deep within Juzo, and a sinister, hooded figure with a scarred face begins to appear around him. This figure calls himself Number 13.

As Juzo struggles to maintain control, he is haunted by vivid flashbacks of his school days where Akai and his gang forced him to eat bugs and eventually threw acid in his face in a science classroom. His friendly coworker, Seki Hajime, tries to reach out to him, but becomes suspicious of Juzo's erratic behavior. When Seki attempts to warn Akai, he is confronted and killed by the elusive Number 13, who then continues to stalk and terrorize the apartment building. The violence escalates as Juzo finds Nozomi murdered in her home and realizes the toddler has been taken to their abandoned middle school.

The story culminates in a brutal confrontation at the school. Juzo discovers the child has drowned, and he is attacked by a vengeful Akai. In the ensuing fight, Juzo overpowers his tormentor. As he is about to deliver a final, fatal blow with a shard of glass, Akai screams, I am sorry. This apology fractures the narrative into two separate realities. In the first, the cycle of violence continues. In the second, a middle-aged Juzo walks through a peaceful city. In this world, the original schoolyard acid attack never happened because a younger Juzo finally stood up for himself, forcing a sincere apology from the young Akai. The apartment building where they lived is being demolished, suggesting that the traumatic events of the first timeline have been erased. As Juzo walks away with Akai as his friend, he glances at a window in the condemned building and sees the dark, hooded figure of Number 13 staring back before he turns away and continues down the street.