After securing a grant to study stellar structures, American applied mathematician David Sumner moves with his glamorous young Cornish wife Amy to a house near to her home village of Wakely in the Cornish moorland. Amy's ex-boyfriend Charlie Venner, along with his cronies Norman Scutt, Chris Cawsey, and Phil Riddaway, immediately resent that the meek outsider has married one of their own. Scutt, a former convict, confides in Cawsey his jealousy of Venner's past relationship with Amy. David meets Venner's uncle, Tom Hedden, a violent drunkard whose flirtatious teenage daughter Janice seems attracted to Henry Niles, a mentally deficient man despised by the entire town.
The Sumners have taken an isolated farmhouse, Trenchers Farm, that once belonged to Amy's father, and still contains his furniture. They hire Scutt and Cawsey to re-roof its garage, and when impatient with lack of progress add Venner and his cousin Bobby. Tensions in their marriage soon become apparent. Amy criticizes David's condescension towards her and his escape from the volatile, politicized campus, suggesting that cowardice was his true reason for leaving America. He responds by withdrawing deeper into his studies, ignoring both the hostility of the locals and Amy's dissatisfaction. His aloofness results in Amy's attention-gathering pranks and provocative demeanor towards the workmen, particularly Venner. David even struggles to be accepted by the educated locals, as shown in conversation with the vicar, Reverend Barney Hood, and the local magistrate, Major John Scott.
When David finds their missing cat hanging dead in their bedroom closet, Amy reckons Cawsey or Scutt is responsible. She presses David to confront the workmen, but he is too intimidated to accuse them. The men invite David to go hunting the following day. They take him to a remote location and leave him there with the promise of driving birds towards him. With David away, Venner goes to Trenchers Farm where he attempts to force Amy sexually. What starts off as rape eventually turns consensual. After, Scutt enters silently, motions Venner to move away at gunpoint and rapes Amy, who responds less passionately, while Venner reluctantly holds her down. David returns much later, smarting from the practical joke the men pulled on him. Amy, though clearly upset, says nothing about the intruders and what they did to her, apart from a cryptic comment that escapes his attention.
The next day, David fires the workmen, ostensibly for their slow progress. Later, the Sumners attend a church social where Amy becomes distraught on seeing her rapists. At the social, Janice invites Niles to leave with her and she begins to seduce him away from the crowd. When it is discovered that Janice is missing, her brother is sent to search for her, and as he calls out for her, Niles panics and strangles Janice to death. The Sumners leave early, driving through thick fog, and accidentally hit Henry Niles as he is escaping the scene of the crime. They take him to their home and David phones the local pub to report the accident. The locals, who in the meantime have learned that Janice was last seen with Niles, are thereby alerted to Niles's whereabouts. Soon, Hedden, Scutt, Venner, Cawsey and Riddaway are drunkenly pounding on the Sumners' door. Inferring their intention to lynch Niles, David refuses to let them take him, despite Amy's pleas. The standoff seems to unlock a territorial instinct in David: "I will not allow violence against this house."
Scott arrives to defuse the situation, but is accidentally shot dead by Hedden during a struggle. Realizing the danger to him in witnessing this homicide, David improvises various traps and weapons, including boiling oil, to fend off the attackers. He inadvertently forces Hedden to shoot himself in the foot, knocks Riddaway unconscious and bludgeons Cawsey to death with a poker. Venner holds him at gunpoint, but Amy's screams alert both men when Scutt assaults her again. Scutt suggests Venner join him in another gang rape, but Venner shoots him dead. David disarms Venner and in the ensuing fight snaps a displayed mantrap around Venner's neck, killing him. Reviewing the resulting carnage and surprised by his own violence, David mutters to himself, "Jesus, I got 'em all." A recovering Riddaway then brutally attacks him, but is shot by Amy as he tries to break David's spine.
David gets into his car to drive Niles back to the village. Niles says he does not know his way home; David says he does not either.
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