Christina Tarling is a teenager who moved from Los Angeles, California to a small town in Avendon, Washington with her father, James Tarling and younger brother Bobby Tarling to be closer to Joanne Tarling, Christina's mother who, due to her mental state, was placed into a psychiatric hospital in Avedon. Christina is shown to be a popular student, and because of his work commitments, James is often out of the house, giving Christina the opportunity to spend time with her boyfriend Eddy Duncan, whom James is not fond of.
Over the days that follow, Christina begins to notice bizarre occurrences happening around her home, especially in the attic, where their Handyman, Howie Rhodes, is constantly inspecting and repairing faults in the house. Sheriff Sheriff Mark Sklar is investigating a missing girl and questions Christina, Bobby, and Karen, Christina’s friend. Michelle, a classmate of Christina and Eddy’s ex-girlfriend who dislikes Christina, is sent a note allegedly from Eddy claiming he ended things and to meet him outside Christina’s house. Outside, Michelle hears a male voice coming from the woods claiming to be Eddy, though the figure remains unseen. Days later, Christina, Karen, and Bobby discover Michelle’s decomposing and dismembered body in a river outside the house.
Eddy grows jealous of Howie’s bond with Christina, which leads him to physically assault Howie. On her 17th birthday, James plans a surprise party with Eddy and all her friends in attendance. During the party, Eddy and Christina sneak away into the attic to get intimate, but James interrupts them. After an argument between James and Christina, James notices a gift hidden away from the others, and when he opens it, he finds a dead rat caught on a mousetrap inside. Startled, James leaves the party. Karen, upstairs, hears noises from the attic and goes to investigate before being dragged away by an unseen man.
After visiting her mother, Christina begins to suspect that James might have had something to do with her mother’s condition, which he denies. Christina begins to hear noises around the house, including a Boombox playing. Christina tries to exit the house with Bobby but finds they are trapped inside. Eddy arrives, and Howie emerges from the attic, revealing he’s behind all the occurrences under Joanne Tarling’s orders of “protecting” Christina, a figure he says he encountered at the psychiatric hospital when he was a patient.
Eddy attempts to stand his ground, but Howie overpowers him with one hand and murders him by impaling the side of his head with a maul, then giving chase to Christina. James arrives at the house but falls into a modified trap rigged with circular saw blades, severely wounding him. Howie catches Christina and takes her to the attic, where he stores the bodies of Karen and the missing girl the sheriff is searching for. James, having escaped the trap, intervenes but is stabbed by Howie. Christina and Bobby set the same trap used on James, then lure Howie out, causing him to fall into it. Christina then pushes a dresser into the trap, crushing Howie to death.
James, having survived, lies in the hospital recovering with Christina and Bobby by his side. After a brief conversation with Sheriff Sklar, Christina and Bobby decide to walk home. At the psychiatric hospital, a newly released patient notices Joanne sitting a distance away. Joanne begins chanting, using the same method she had used on Howie to control him, and she smiles at him as he leaves the building. Later that evening, the lights in the attic at Christina’s house turn on, indicating the man has assumed Howie’s former role.
