A young psychotic girl murders her parents with a hammer in the middle of a stormy night.
Four years later, a newly divorced woman, medical doctor Sarah Cassidy (Elisabeth Shue), and her 17-year-old daughter Elissa (Jennifer Lawrence) move to a small, upscale town. Their house is near the house where the massacred family lived. As told by the neighbors, four years prior, a girl named Carrie-Ann Jacobson killed her parents, then fled into the forest and was never seen again, leaving her brother Ryan (Max Thieriot) as the sole survivor. Ryan now lives alone and is hated by his neighbors; Bill Weaver (Gil Bellows), a local police officer, appears to be Ryan's only supporter.
The mother-daughter relationship becomes rocky and Elissa starts dating Ryan against her mother's wishes, finding Ryan to be lonely but a sweet boy. Ryan confides in her that he accidentally injured Carrie-Ann by allowing her to fall from a swing when they were little; he was supposed to be watching her while their parents were getting high on drugs. The resulting brain damage from the accident made her extremely aggressive, leading to their parents' murder. Ryan is revealed to have been secretly taking care of a seemingly now-grown Carrie-Ann (Eva Link) in a hidden room. When she, Ann, escapes, Ryan accidentally kills her while trying to hide her. In grief, he goes to the diner, where he meets a kind waitress named Peggy (Jordan Hayes).
Later, some unruly high school boys pick a fight with Ryan and he flees, and Elissa drives to his house and subdues a fire the boys started. She finds tampons in the kitchen garbage and explores the house with suspicion until she finds the secret room and is attacked by Carrie-Ann, who is revealed to actually be Peggy. Ryan restrains "Carrie-Ann" while frantically screaming at Elissa to leave. Elissa finds blue contact lenses and Peggy Jones's wallet in the kitchen. Ryan has kidnapped the waitress and attempted to make her look like Carrie-Ann. When Elissa tries to leave, Ryan knocks her out by hitting her head on the door.
Elissa wakes to find herself tightly tied to a chair. Ryan reveals that Carrie-Ann actually died during the swing accident. He says his parents blamed him for it, and implies that he was the one who killed them. He explains that he wants Elissa, but that he needs Carrie-Ann and cannot have both. Officer Weaver goes to Ryan's house to look for Elissa, but Ryan stabs him to death. Elissa frees herself and tries to escape, but Ryan knocks her out with Chloroform and traps her in his car trunk with Peggy's body. Sarah arrives and is also stabbed by Ryan. Elissa struggles out and ultimately shoots Ryan with Weaver's gun. When she approaches him, he suddenly wakes up and grabs her wrist. When Ryan attempts to stab Elissa with the knife, Sarah strikes him in the head with the hammer.
Elissa and Sarah move out; Ryan is placed in a psychiatric ward. A flashback shows young Ryan about to blow out birthday candles. His mother calls him "Carrie-Ann" and when Ryan protests that his name is Ryan, not Carrie-Ann, she slaps him violently; his parents had forced him to dress and act like Carrie-Ann after she died and most likely abused Ryan when he refused to go along with their fantasy, thus setting Ryan on his troubled path.
Who wakes to find herself tightly tied to a chair?
Elissa