When teenage friends Libby Mannering (Andi Garrett) and Kit Austin (Sara Lane) are home alone with Libby's younger sister Tess (Sharyl Locke), they amuse themselves by randomly dialing telephone numbers asking prank questions, telling whomever answers: "I saw what you did, and I know who you are." Libby places a call to Steve Marak (John Ireland), a man who had just murdered his wife Judith (Joyce Meadows) and disposed of her body in the woods. Believing he has been found out, Marak wants to track down the caller in order to silence her.
Marak's neighbor Amy (Joan Crawford), who is in love with him and had been trying to woo him away from his wife, listens in on the line while Marak is speaking with Libby. Intrigued by Marak's voice, Libby takes Tess and a frightened Kit along on a drive to Marak's address. Amy discovers Libby and chases her off, thinking that Libby is Marak's younger, inadvertently saving Libby from being murdered by Marak, who has seen her and grabbed a knife. Amy snatches the registration card from the car before Libby drives away and gives it to Marak, telling him to keep it as a souvenir of his last fling. Amy tries to blackmail him into marrying her, telling him she knows about the murder, but he stabs her to death after they have a drink. With Libby's address and phone number from the car registration, Marak calls to ask if her parents are home and then sets out to the Mannering home.
Libby's mother, 90 miles away in Santa Barbara with her husband, is frantic with worry when no one answers the home phone and has her husband call the police to check the house. A patrolman visits and finds that the three girls are safe.
Libby, afraid of losing her driving privileges, swears Kit to secrecy over their misadventure. While Kit's father is driving her home, a news report over the car's radio announces that a woman's body has been found in the woods and provides a description of a man who was seen leaving the burial site.
Marak arrives at the Mannering home and questions Libby and Tess about the call. Libby convinces him that it was just a prank. He returns her mother's identification and leaves, but waits outside. When Kit calls, she tells Libby that Marak matches the description of the killer about whom she had heard on the radio. Marak overhears and enters to silence Libby and Tess, but they evade him. Libby tries to escape but cannot start her parents' car. Marak emerges from the back seat and starts to strangle Libby, but he is shot by a police officer who had come back to the Mannering house after Kit revealed the secret to her father, who phoned the police.
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