The film begins with a blurry scene that gradually reveals a group of schoolchildren playing in a cemetery located in the secluded village of Wakely, along the coast of England. American mathematician David Sumner takes a stroll down the road, accompanied by his new wife, Amy Sumner, who provocatively walks braless while holding a daunting device known as a 'mantrap'. It’s here that David encounters Charlie Venner, portrayed by Del Henney, an intense man with a past romance with Amy. David also meets Janice Hedden (Sally Thomsett) and her younger brother Bobby (Len Jones).
While loading the mantrap into the car, David shares that he has come to Wakely on sabbatical to write a treatise on celestial navigation. To maintain the tranquility he desires for his writing, David decides to hire Charlie and his friend to assist with repairs on their new home. Later, David visits a local pub where he meets the gruff bartender Harry Ware, asks for American cigarettes, and witnesses an altercation between Janice's father, Tom Hedden (Peter Vaughan), and Harry. The confrontation escalates into a brawl before David chooses to leave, declining Tom’s offer to buy him a drink.
As David and Amy drive to their new residence, they encounter Norman Scutt, who is busy fixing the garage and shortly after, the giggling Chris Cawsey (Jim Norton), who specializes in exterminating rats. Both men are aware of the social unrest and escalating violence that defined the Vietnam era in America, which David seems to be trying to escape. His obsession with his research leads him to treat Amy and even her cat poorly, which greatly frustrates her. One day, during a quarrel over his neglect, Amy decides to show her discontent by parading topless in view of the workers.
As tension builds, Amy discovers her missing cat and, while searching outside, finds David harassed by the workers and their contempt towards him intensifying. The pub scene grows more hostile, with David witnessing a strained relationship between Janice and a suspected predator, Henry Niles, a character embroiled in controversy. David and Amy's domestic life becomes increasingly tumultuous, culminating in Amy's concern when she finds her cat dead, further straining their marriage.
After an unsettling evening spent at a church function where Amy is haunted by the memory of her earlier violation, David becomes embroiled in a surreal battle for survival. Misunderstandings lead to violence when the workers, emboldened by alcohol and rage, descend upon David's home in search of Henry.
Just as David resolves to protect his new residence and wife, chaos ensues—he recalls the importance of standing firm: > "This is where I live. This is me. I will not allow violence against this house." With unyielding determination, David defends their home using every tool at his disposal, confronting each intruder with ferocity.
Ultimately, in a fevered climax, bloody battles unfold as David struggles against overwhelming odds. When the dust settles, he is left to grapple with the savage reality of the violence he became part of. Amidst the aftermath, David reveals to Henry, "I don't know my way home," leaving the audience to ponder the inescapable consequences of their harrowing choices. The transformation from a mathematical scholar seeking solitude to a man embroiled in a brutal fight for survival epitomizes the film's stark exploration of human nature and violence.
