Steven is a 25-year-old man living in a decaying flat with his mother, a grotesquely abusive woman he calls the Hagbeast. He did not leave the apartment until age five, and the Hagbeast so thoroughly eroded his capacity for independence that by his teens, existing in the outside world seemed unthinkable. She feeds him deliberately rancid meals he believes are slowly poisoning him. His only companion is Dog, a loyal animal whose back legs were paralyzed years earlier when the Hagbeast struck it with a brick. Television is Steven's sole window into a world of love and belonging he desperately longs to enter.
Steven begins working at a meat grinding plant. His foreman, Cripps, assigns him to the grinder but reveres the slaughter room, where an elite team kills cattle. Cripps claims killing frees a person from weakness and reveals true power. Steven also meets Gummy, a disfigured worker whose lips and teeth were bitten off by a cow he had been kissing.
Returning home, Steven meets Lucy, a woman of half-Indian, half-Jewish heritage who lives in the flat above his. Lucy channels her alienation into an obsessive conviction that a hard, black deposit of poison accumulates inside the body from psychological damage. She keeps surgical instruments and watches operation videos, searching for evidence of the black mass she believes doctors conceal from patients. She begs Steven to look inside the organs of slaughtered cows, and when he finds nothing, she enlists his help using an endoscope, a flexible camera probe, to examine her own colon. Their search is inconclusive, but they begin a sexual relationship. Steven starts to see Lucy as the wife and mother around whom he can build the television-modeled family he craves.
Steven recognizes the Hagbeast will destroy any source of hope she discovers and concludes she must die before she learns about Lucy. Direct violence feels impossible after years of conditioned submission, so he devises an indirect method: He will take over the cooking and feed her his own feces. When the Hagbeast refuses to yield, Steven punches her in the head for the first time in his life, stunning her and seizing control of the meals.
Meanwhile, Cripps draws Steven deeper into the slaughter room. Steven fires a boltgun, a captive-bolt device used to kill cattle, into a cow's head while Cripps presses behind him, anally penetrates him, and ejaculates at the moment of the kill. Steven later watches the slaughtermen sexually violate a living cow and blacks out.
He wakes in the dark slaughter room surrounded by cattle. A Guernsey, a breed of dairy cow, speaks to him. Steven recognizes the animal as the one whose eyes he glimpsed through a ventilation grille. The Guernsey warns that Cripps' philosophy is fraudulent: The slaughtermen appear transformed not through enlightenment but because habitual killing has deadened their ability to feel. The Guernsey carries Steven through ventilation ducts into a vast underground network of sewers and tunnels beneath the city. At its center, roughly 200 cows live freely, having escaped the plant years earlier. The Guernsey asks Steven to lure Cripps to the slaughter room alone so the herd can kill him. Steven refuses.
Back at the flat, Steven serves the Hagbeast plates of his feces. She vomits but forces herself to eat. The meals become a grotesque contest of wills; the Hagbeast grows visibly ill but taunts Steven that he lacks the strength to kill her.
Cripps escalates further. He presents Gummy bound in a grabber, a pneumatic frame used to immobilize cattle. Using secateurs, heavy pruning shears, Steven cuts Gummy open from rectum to skull at Cripps' direction. Steven collapses, then stumbles to Lucy's flat. The next morning he wakes free of fear and realizes he can kill the Hagbeast directly.
He goes downstairs for a knife, but the Hagbeast ambushes him from the darkened kitchen, slamming him to the floor and looping a rope around his neck. As his vision fades, Dog drags itself down the hall and bites the Hagbeast's throat. She rips Dog free and smashes its head against the wall, killing it. Dog's eyes hold Steven's gaze through the final arc.
Dog's death unleashes Steven's rage. He overpowers the Hagbeast, binds her in the kitchen, extracts her teeth with pliers, stuffs her nose with toilet paper, and tapes her open mouth against his anus. He defecates down her throat until she suffocates. Steven fetches Lucy, and they move her belongings in. Lucy dissects the corpse searching for poison deposits but finds nothing, clinging to Steven in despair. Steven burns the remains on the roof.
For several weeks, the couple remodels the flat to resemble the homes Steven has seen on television. Lucy is pregnant, and Steven believes the child will complete his dream. But his confidence erodes, and after days of mounting anxiety he identifies what he needs: another killing.
Steven returns to the plant and agrees to kill Cripps for the underground herd, insisting on doing it himself. He lures Cripps to the slaughter room, shoots the boltgun into his knee, and drags him to the underground chamber. Before the assembled herd, Steven strips the flesh from Cripps' limbs with a flensing knife, a powered blade used for skinning, and continues until Cripps dies. The herd kneels before Steven.
Steven assumes leadership, delivering messianic speeches and leading violent stampede raids through the city's underground infrastructure. The cows kill workers and commuters as Steven collects cash. He cements his authority by mating publicly with a small roan cow who offers herself and later feeding the herd human flesh as a sacrament. The Guernsey accuses Steven of becoming another Cripps and positions itself to seize control.
At home, Lucy confesses she still feels the poison building inside her. Steven insists the baby will fix everything. One day, alone in the flat, Lucy stops believing she carries a child. To her, the mass in her belly is a stone of crystallized poison. She cuts open her lower abdomen with a scalpel, reaches into her womb, and pulls the fetus free. She dies feeling clean.
Steven finds Lucy dead on the kitchen floor. His constructed world collapses. He nails the fetus to the wall, then lies catatonic for three days. He drags Lucy's body and Dog's remains to the roof. Days of rain cause the building's back wall to collapse, and emergency services arrive. Steven escapes through the yard.
Stripped of every refuge, Steven runs through morning streets until each passerby seems to corrode his identity. He tears open a drain grate and falls into the sewer. Hours later, he navigates the tunnels by instinct to the herd's central chamber, which is empty. Steven retrieves Cripps' thigh bone from the skeleton behind the mound and sharpens it. When the herd returns from a stampede led by the Guernsey, Steven waits for the cows to sleep, then drives the splintered bone through the Guernsey's neck.
Steven stands over the body with arms spread, feeling godlike. The roan cow nuzzles his side. He sees his future: He will live underground with the herd, free of comparison with the outside world, surrounded by creatures that worship him. He fills his lungs to shout. It is time to wake the herd.