Christie Cromwell, a teenage girl in New Rochelle, New York, discovers that her stepfather Paul Fox is trying to murder her and her mother Karen Cromwell for her money, but when she tries to tell others, no one believes her. After a maintenance worker dies in the basement, Christie suspects that Paul set the trap for her mother after she saw him leaving the basement the night before.
Christie begins shadowing Paul and uncovers that he is having an affair with a young woman named Brenda Bohle. She is caught by Lacey Bohle but manages to flee. Paul then convinces Karen that Brenda is merely a client of his, and Karen chooses to trust him over Christie.
To prove the danger, Christie enlists the help of Josh Dealey, who is the boyfriend of her best friend Janey Ralston, and the two set out to confront Paul. But danger escalates when the brakes on Christie’s mother’s car are tampered with, nearly causing a fatal accident. Christie and Janey track Paul to a motel, where they try to escape as Paul spots them; soon after, Janey is killed in a hit-and-run by an unseen driver. Christie goes to the police and claims Paul killed Janey, but she is dismissed as unreliable, leaving Josh to stand by her in the aftermath.
Later, Christie and Josh share a private moment, but their closeness is interrupted when Paul orders Josh to leave. In the ensuing chaos, Christie discovers gas in the house and realizes Paul intends to kill her as well. Karen also falls down the stairs after a trap Paul has set, leaving her wheelchair-bound. Christie then takes a photo of Paul with Brenda, but drops it and is seen; while retrieving it she learns that Brenda and Lacey are actually a married couple who plan to blackmail Paul after killing Karen and Christie. Christie shows the photo to her mother, prompting Paul to leave the house.
Midnight brings a terrifying confrontation as Paul, Brenda, and Lacey invade the home, forcing Christie and Karen to the basement. They reveal plans to kill them at 2 a.m. and pin the crime on a burglar. Christie reveals the true relationship between Brenda and Lacey, enraging Paul. She tricks Brenda into allowing her into the bathroom, while Karen cuts the electricity from the basement, creating a window for escape. Christie runs and manages to stab both Lacey and Paul. Brenda attacks Karen and then turns the lights back on, but Christie surrenders to Lacey, and the two are driven back to the basement, where they improvise another escape by wetting the fuse box.
At 2 a.m., the plan is foiled by Josh’s knock at the door. He is suspicious and contacts the police as the electricity goes out, allowing mother and daughter to flee. Lacey orders Brenda to restore power, but the wet fuse box shocks Brenda to death. Lacey rushes to the basement after hearing Brenda’s scream and finds her dead. Christie then tricks Paul into thinking she’s in the bathroom; when Paul enters the gas-filled space with a lighter, the bathroom explodes and he is killed. Josh saves Christie from the burning house.
With the danger over, Christie and Karen relocate temporarily until Josh visits and kisses Christie. Lacey appears, knocks Josh out, and resolves to finish Christie for Brenda’s fate. Christie defends herself with a knife and stabs Lacey through the stomach, killing him.
