During a relentless downpour, a woman named Emma Parker stumbles into a store just as the butcher is receiving a delivery. After several attempts to communicate with her, she confesses that the harsh weather has deeply affected her. The butcher warns her that conditions will worsen before they improve, and she proceeds to buy groceries. As Emma exits the store, she notices a poster about missing teenage girls. Before she can reach her car, she starts bleeding from her mouth and loses consciousness after hitting her head on a structure, ultimately drowning in a rain-filled ditch.
Later, devastation strikes the Parker family as the sheriff informs Frank Parker of Emma's death. Overwhelmed with grief, Frank abstains from identifying her body, sending his daughters, Rose and Iris, in his place. Doctor Barrow, who had delivered Frank's young son Rory, announces that an autopsy is required by law. During the autopsy, he discovers signs of Kuru disease. Meanwhile, Frank is offered comfort by his compassionate neighbor Marge, but later, while navigating through the storm, he encounters a stranded motorist, strongly implying that he attacks her with a tire iron.
Interwoven into the narrative are scenes from a book being read by the eldest daughter, chronicling their family's pioneer history, filled with tales of hunger and hardship. The men of their party have been lost in the wilderness, leaving the women facing a grim winter. Rose and Iris contemplate taking over their mother's religious duties; however, Iris insists that they must follow this year's ritual. Rory grapples with understanding their family's tumultuous past.
In a shocking turn, Rory accidentally discovers a young woman held captive in his father's shed. Frank, furious, instructs Rory to leave and commands his daughters to butcher the captive. Though they resist, they follow his orders under duress and eventually consume her remains at Frank's insistence. Meanwhile, Marge attempts to deliver a vegetarian meal to the Parkers but is met with a cold reception from Iris, who brushes off Marge's concern about strange noises from the shed.
Through flashbacks to pioneer times, the family's struggle for survival is depicted as they seek refuge in a cave. Meanwhile, Frank's sanity deteriorates as he becomes increasingly withdrawn, disturbing his daughters with his nightly cries. He eventually reveals their meager food supply: a woman from their group tied to a rack, from whom he has been cutting pieces. Handing his eldest daughter a knife, he points towards the carcass, leaving her with a grim decision to make.
Doctor Barrow, whose daughter has also gone missing, grows suspicious upon discovering a bone fragment in a creek. Although Sheriff Meeks dismisses his worries, Barrow persuades Deputy Anders to investigate further. They uncover additional evidence, leading to a tense encounter with Iris in a secluded area. As Anders confesses his feelings, Iris, riddled with guilt, breaks down. Their emotional moment escalates into a sexual encounter, only to be abruptly halted when Frank catches them and kills Anders in a fit of rage, commanding Iris to return home.
With Frank now ordering his daughters to remain confined to their room, Rose devises an escape plan. Frank, alone in his room, whispers prayers of devotion, convinced they will soon be reunited with their mother. As he prepares dinner, he adds a suspicious jar of powder to the soup. Realizing Frank's intent to poison them with arsenic, Rose attempts to alert Iris but is unsuccessful. In a desperate measure, she knocks over Rory's bowl to prevent him from eating before Frank can react.
Just then, Barrow arrives, armed with findings that imply the Parkers have engaged in acts of cannibalism. As he confronts Frank about the missing Anders and his daughter, the tension escalates, culminating in a chaotic struggle where Iris shields Barrow from Frank's gunfire, resulting in her accidental injury. In the heat of the moment, Barrow shoots Frank, but Frank ultimately overpowers him.
Amid the turmoil, Rose and Rory flee to Marge's house for shelter. Tragically, Frank breaks in, kills Marge, and then manipulates Rose and Rory to return to him. Back at their home, Frank coerces them into eating again, comparing Rose to her deceased mother. In a shocking turn of events, Rose retaliates by biting into Frank's neck, while Iris drives a knife through her father's hand. Together, they launch an assault on him until he succumbs.
As the grisly scene unfolds, Rose notices that Barrow, barely conscious, has witnessed the entire incident. In a haunting gesture, she places his daughter's stolen hair ornament on his chest. The following morning, the siblings depart from town, with Rose clutching a diary that recounts their ancestors' grim history of cannibalism.
