Dave and Ellie Mannering are preparing to leave for an overnight trip when their babysitter becomes ill. They reluctantly leave the responsibility for the house and 9-year-old Tess to her teenaged sister, Libby. Libby's friend Kit joins them, and they begin a telephone game in which they call people at random and ask silly questions. One of the numbers they find belongs to Steve Marak, whose wife, Judith, is about to leave him. Judith answers Libby's call and tries to get Steve out of the shower, but their continuing argument becomes so intense that he murders her. Steve takes the body into a wooded area and buries it. He returns to find Amy, his widowed neighbor who is in love with him, waiting at the house. Libby calls Steve again, using the name Suzette, and says, "I saw what you did, and I know who you are." Steve begins to panic and tries to counter Libby's quips with his own questions concerning a rendezvous. Amy becomes jealous while overhearing the conversation and goes into the bathroom; there she finds Judith's bloody clothes and realizes what has happened. The girls, curious to see Steve, visit him; but Amy sends them away after pocketing their automobile registration. Steve kills Amy when she tries to blackmail him into living with her; and, taking the car registration, he goes to the girls' home. Kit's father picks her up, and she eventually tells him the story. He calls the police, and they arrive at the Mannerings' home in time to shoot Steve before he strangles Libby.
