Haute Tension

College girlfriends Marie (Cécile de France) and Alex, or Alexia (Maïwenn Le Besco), are on their way to stay at Alex's parent's house for the weekend. When they arrive, Alex gives Marie a tour of the 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 responding. The man at the door is a serial killer (Philippe Nahon), who slashes Alex's father and decapitates him. Marie hears all this from her bedroom. She arranges her room to make it appear that no one is staying there, and hides under her bed. The killer inspects Marie's room but doesn't find her. Marie creeps downstairs and finds Alex chained in her bedroom. Promising to find help, she sneaks into the parent's room to find a phone. After hearing loud thuds, she hides in the closet, and witnesses the killing of Alex's mother. Alex's younger brother escapes the house but is followed to the fields by the killer and shot. Alex is dragged into the killer's truck. Marie sneaks into the truck with a butcher knife and hides there with Alex.When the killer stops at an all-night gas station to refuel his small truck, Marie hands Alex the knife and sneaks into the gas station before him. While hiding behind a row, she witnesses the killer talk for a while to the convenence store clerk before paying for the fuel, and then when the clerk goes to one of the locked shelves to aquire a bottle of whisky that the killer asks for, the killer viciously kills the clerk with a fire axe. Marie hides in a downstairs restroom while the killer goes to releave himself. After the killer drives away, Marie tries calling the police, but becomes frustrated and hangs up.Marie takes the clerk's car keys and chases the killer down a deserted road. The killer notices Marie following him, and rams into Marie's vehicle, pushing her car off the road injuring her with a minor head wound. Exiting on foot, Marie runs into the forest, and the killer follows her. Eventually, Marie mortally wounds the killer with a pole that has barbed wire wrapped around it. As Marie inspects the body, he grabs at her throat, so Marie suffocates him with a plastic sheet. After that, she makes her way back to the truck.Meanwhile, the police investigating the gas station murders view the in-store videotape... and see Marie murdering the store clerk. 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/The Killer 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. Marie shoves the concrete saw through the wind shield 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 kisses her. While engaged in the kiss, Alex lunges the crowbar into the killer's chest and through her back while the killer proclaims how she'll never let anyone come between them.The final scene shows the scarred Marie in a psychiatric hospital room, and Alex watching her through a one way mirror. Marie grins and reaches for Alex, aware that she is behind it.

Question: Who is chained in her bedroom?
Answer: Alex