I Saw What You Did is what I would classify as a 1965 American suspense, thriller film rather than the genre of horror. The film was released by Universal Pictures and starring Joan Crawford and John Ireland. The plot follows two teenage girls who find themselves in serious danger after making a prank phone call to a man who has just murdered his wife. The screenplay by William P. McGivern was based upon the 1964 novel Out of the Dark by Ursula Curtiss. The film was produced and directed by William Castle.
When two mischievous teens Libby (Andi Garrett) and Kit (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 has recently murdered his wife, Judith (Joyce Meadows) and disposed of her body in the woods. Believing he has been found out, he decides to track down the caller to silence her.
Marak’s neighbor Amy (Joan Crawford) is in love with him and has been trying to woo him away from his wife. She finds out about the murder. Libby fatefully decides to get a look at Marak because she was intrigued by his voice and takes an increasingly frightened Tess and Kit in her parent’s car to Marak’s address.
Amy discovers Libby and chases her off, thinking she’s preventing Marak from meeting with a younger lover but inadvertently saving the girl from being murdered by Marak, who has seen her and grabbed a knife. Amy also snatches Libby’s mother’s car registration from the car seat before Libby drives away, and gives it to Marak, telling him to keep it as a souvenir, his last “Suzette” meaning involving himself with a much younger woman like his deceased wife, Judith. Amy tries to blackmail him into marrying her, telling him she knows about his wife Judith, but he stabs her to death after they have a drink. Libby’s mother’s identification has the family address and phone number, which Marak then uses to track down the girls. He calls asking if her parents are home. She innocently answers no, and he sets out to her home.
During this time the parents have been unable to contact the girls by phone. A policeman arrives at Libby’s home to investigate just after the girls arrive at the house. Libby swears Kit to secrecy over their misadventure. Kit’s father arrives to take her home. While he drives her home, the car radio announces that a woman’s body was found in the woods with a description of the man seen leaving the burial site.
Marak enters the home and questions Libby and Tess about the call. Libby convinces him it was just a prank. He returns her mother’s identification and leaves but waits outside. Kit calls and Libby describes Marak. Kit tells her that he matches the description of the killer just reported on the radio. Marak overhears this and enters to silence Libby and Tess but they evade him. Kit tells her father and he calls the police. 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. Libby and Tess return to their home to await their parents’ return from Santa Barbara.