Best friends Marie and Alex are on their way to stay at Alex's parents' house for the weekend to study. Upon arriving, Alex gives Marie a tour of her house, then they have dinner and go to bed. While the family sleeps, Marie stays awake in the guest bedroom, listening to music and masturbating. Marie hears a doorbell ring and Alex's father Daniel answers it. A man slashes Daniel's face with a straight razor, presses him between two staircase spindles, and shoves a bookcase towards his head, decapitating him. Alex's mother, awakened by the noise, finds Daniel dead and is approached by the killer. Marie, hearing the mother's screams, rearranges the guest bedroom to conceal her presence, and hides under her bed. The killer inspects Marie's room but does not find her. Marie creeps downstairs and finds Alex chained in her bedroom. Promising to find help, Marie sneaks into the parents' room to find a phone. After hearing loud thuds, she hides in the closet and witnesses the killer murdering Alex's mother by slashing her throat with a razor. Alex's younger brother Tom runs from the house to the cornfield, pursued by the killer. Marie returns to Alex, where she witnesses Tom's murder from a window. Marie promises to free Alex, but the killer is heard returning. Marie sneaks into the kitchen and takes a
butcher knife. Alex is dragged into the killer's truck while Marie sneaks into it and hides there with Alex. He locks them in and drives off. When the killer stops at a gas station, Marie gives Alex the knife and sneaks in for help. Inside, Marie witnesses the store clerk being murdered with an axe. As the killer drives away, Marie calls the police but hangs up when she's unable to tell them where she is. Marie takes the clerk's keys and uses his car to follow the killer down a deserted road. Noticing Marie tailing him, the killer rams Marie's vehicle, pushing the car off the road. Badly injured, Marie runs into the forest as the killer seeks her. Eventually, Marie bludgeons the killer with a fence post covered in barbed wire. As Marie inspects the body, he grabs at her throat, so Marie suffocates him with a plastic sheet. As Marie makes her way back to the truck, Alex seems terrified of her. Meanwhile, police at the gas station watch security camera footage, which shows Marie murdering the store clerk. It is revealed that Marie is murderous, delusional, and obsessed with Alex, being responsible for all the murders. At the truck, Marie unties Alex. As soon as Alex is free, she threatens Marie with the knife and accuses her of butchering her family. Alex slashes Marie's face and stabs her in the stomach before running into the forest. Marie chases Alex with a
concrete saw. Alex finds a road and flags down a car. As Alex is climbing into the car, Marie appears brandishing the concrete saw and disembowels the driver. A stray piece of glass slices Alex's
Achilles tendon. Alex takes a crowbar from the car's toolbox and crawls along the road. Marie forces Alex to tell her that she loves her, and she kisses her. While engaged in the kiss, Alex plunges the crowbar into Marie's upper-chest as Marie proclaims she'll never let anyone come between them. Sometime later, Marie is in a
psychiatric hospital room chained to her bed, with Alex watching her through a
one-way mirror. Marie grins and reaches for Alex, seemingly aware that she is behind it. ==Cast==