High Tension

Marie and Alex are best friends on their way to stay at Alex's parents' house for the weekend to study. When they arrive, Alex gives Marie a tour of her house before they settle down for dinner. After dinner, Marie and Alex get ready to go to bed. As Alex sleeps, Marie lies on her bed listening to music and masturbating. Marie hears a doorbell ring and Alex's father Daniel wakes to answer it. The man at the door is a serial killer, who slashes Daniel's face with a straight razor. His head is pressed between two spindles of the staircase, and the killer shoves a bookcase towards his head, decapitating him. The noise awakens Alex's mother, who finds Daniel dead and is approached by the killer.

Marie, hearing the mother's screams, quickly arranges the guest room to make it appear that no one is staying there, 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, she sneaks into the parents' room to find a phone. After hearing loud thuds, she hides in the closet and through the slats of the door witnesses the killing of Alex's mother as her throat is brutally slashed 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. Marie sneaks into the truck with the butcher knife 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 into the gas station shop for help. When the killer comes into the shop, Marie hides and she witnesses the store clerk Jimmy being murdered with an axe. The killer returns to the truck and Marie calls the police, but hangs up in frustration when she's unable to tell them where she is. She takes the clerk's keys and uses his car to follow the killer down a deserted road. The killer notices Marie following him, and rams Marie's vehicle, pushing the car off the road where it wrecks. Exiting on foot, 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 and makes her way back to the truck. Alex seems terrified of Marie as she returns to the vehicle. As police investigate the gas station murders via the in-store videotape, the tape shows Marie murdering the store clerk. In retrospect, it is revealed that Marie is murderous, delusional, and in love with Alex and the real killer of Alex's family.

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.

Some time later, Marie is in a psychiatric hospital room, with Alex watching her through a one-way mirror. Marie grins and reaches for Alex, evidently aware that she is behind it.