Shouta Shamoto is a meek, downtrodden owner of a struggling tropical fish shop, trapped in a tense domestic life with his rebellious
teenage daughter, Mitsuko, and his resentful second wife, Taeko. Their fragile routine shatters when Mitsuko is caught shoplifting at a local supermarket. Instead of pressing charges, the charismatic, eccentric master of the massive "Amazon Gold Fisheries," Yukio Murata, offers to let the incident slide and even employs Mitsuko at his sprawling establishment.Murata quickly insinuates himself into the Shamoto family, showering them with hospitality and manipulating the weak-willed Shouta. However, Shouta soon discovers a terrifying truth hidden beneath Murata’s genial facade: Murata is a ruthless, psychotic serial killer who, alongside his equally complicit wife, Aiko, murders business partners and dismembers their bodies in a horrifying assembly line, dissolving the remains in acid to eliminate all evidence.Trapped by fear, blackmail, and his own crippling cowardice, Shouta is forced to become Murata’s accomplice, helping him dispose of bodies and maintain the gruesome operation. As Shouta is dragged deeper into this nightmare, the psychological torment breaks his psyche. He learns that Murata’s previous victims include people who trusted him completely, and realizes that Murata views human life as utterly worthless, treating murder much like culling diseased fish.The pressure mounts as Taeko, initially disgusted, eventually becomes numb to the violence, participating in the atrocities and further alienating Shouta. Meanwhile, Mitsuko remains oblivious to the dark reality engulfing her father. Desperate to break free, Shouta’s suppressed rage and despair finally reach a boiling point. Realizing that Murata intends to eliminate him and his family once they become liabilities, Shouta snaps.In a shocking and bloody climax, Shouta turns on his tormentors. He brutally attacks and kills Murata in a frenzy of long-repressed aggression,
turning the tables on the sadistic mastermind. He then confronts Aiko, leading to a chaotic, fatal confrontation that leaves the slaughterhouse drenched in blood. With the nightmare finally over, a blood-soaked and utterly broken Shouta returns home. He attempts to reconnect with Mitsuko, but the psychological damage is irreversible. He has transformed from a pathetic, harmless man into a cold-blooded murderer, utterly consumed and corrupted by the darkness he sought to escape.