A group of friends plays volleyball on a sunny Venice Beach in California. Jackie, a high school senior, is set to host a late-night slumber party since her parents are away looking for a new house. After the day on the sand ends, Sarah climbs into her car and is brutally murdered by an unseen assailant in the backseat with a power drill. That night, Morgan arrives at Jackie's house, posing as a potential buyer who wants to tour the interior as he considers purchasing the property.
That evening, Jackie’s friends, Diane, Maria, Juliette, Janine, and Susie, arrive for the party. Soon after, three boys, Frank, Tom, and Michael, show up and startle the group. Jackie sternly tells them to leave, but Frank and Tom go to fetch something for the girls while Michael stays to apologize. Michael encounters a masked killer, who impales him with a “house for sale” sign post. Meanwhile, Duncan bribed a pizza delivery girl to bring pizzas to the house; Pizza Girl arrives, only to be chased and murdered by the same killer.
Back inside, the girls admit Frank and Tom back in, and Ken appears right behind them. Upstairs, Juliette and Ken begin to have sex, but Ken is too drunk; he pleases her orally instead. Ken leaves, and Juliette is electrocuted in the bathtub, later discovered by Maria in a closet. Ken heads out to arm himself, stopping by a lumberyard to gather weapons, and before long, he returns to the house. He kills both Duncan and Frank, and Maria discovers Tom’s dead body before she and Janine flee toward the door. Ken follows, and Maria escapes while Janine leaps through the glass door and is drilled in the back, dying.
Susie hides in an upstairs closet, but Ken attacks her and knocks her unconscious. Maria knocks Ken in the head with a lamp, and he chases Maria, Jackie, and Diane into the basement, where Jackie uses a spear gun to shoot him and they hurry back upstairs. Susie awakens, splashing bleach in Ken’s face and blinding him. He catches Maria and kills her as the trio tries to regroup. Jackie, Susie, and Diane manage to knock Ken unconscious with a croquet mallet.
As they reassess their situation, Ken awakens and begins slashing Diane with a knife, killing her. Susie manages to pin Ken to the floor long enough for Jackie to plunge the drill through his chest multiple times. With Ken dead, Jackie discovers a photo in his shirt pocket showing him as a child with his uncle, a memory that adds a grim layer to the killer’s backstory. Susie goes to answer the police knocking on the door, and the credits roll.
