Tsuda Yoshiharu, Shinya Tsukamoto, a door-to-door insurance salesman, finds a new outlet in boxing after a chance reunion with a former high school friend, Kojima Takuji Kôji Tsukamoto. Under the mounting pressure of supporting himself and Hizuru Kaori Fujii, who quit her job after they became engaged, the world around him grows tense and exhausting.
One day, Kojima Takuji arrives at Tsuda’s apartment, claiming he was invited, and Hizuru lets him in. What begins as a seemingly ordinary visit spirals quickly as Kojima tries to pursue Hizuru, who rebuffs him. Tsuda learns of the encounter and is overwhelmed by anger, confronting Kojima only to be beaten badly and publicly humiliated in front of Hizuru.
Intrigued by Kojima’s feral, animalistic edge, Hizuru breaks up with Tsuda and moves in with his rival. She also begins to pierce herself and get tattoos. Hizuru longs to take up boxing, but Kojima—surprisingly cowardly—denies that life, telling her she is a scary freak of a woman. He is offered a fight against an extraordinarily deadly boxer and accepts only with evident reluctance.
Kojima explains to Hizuru how he first got into boxing. In high school, he and Tsuda shared a mutual friend who was killed by salarymen. The two vow to train and exact revenge when the culprits are released from prison, but Tsuda abandons the plan, leaving Kojima to continue down that path. To Kojima, Tsuda has become “a nobody” who mirrors the men they once despised. Hizuru warns that by provoking Tsuda, Kojima might be awakening a killing machine, though Kojima brushes off the concern and accepts the dangerous fight.
Tsuda’s efforts to win Hizuru back culminate in a brutal confrontation where the two men trade blows to the point of ruin for each other. Hizuru returns home and engages in intense, almost feral sex with Kojima. The boxing club becomes a arena for a sparring session in which Kojima nearly kills Tsuda with a ruthless display of power. While Kojima fights his real boxing match, Tsuda lies in the hospital with a bleeding eye.
Meanwhile, Hizuru’s obsession has intensified, and she has pushed piercings to an extreme, embedding metal bars beneath her skin. Kojima wins the match, but his face is shattered by the brutal aftermath, unrecognizable in victory. Hizuru is shown in a field, attempting to rip out her piercings, only to bleed to death. Tsuda is last seen standing before an apartment building, the pupil of his eye missing. The final image lingers on a lonely punching bag hanging in an empty boxing gym as the cheers of a crowd echo in the distance.
