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). Amy tries to blackmail him into marrying her, telling him she knows about his wife, 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.Joan Crawford as Amy Nelson
John Ireland as Steve Marak
Leif Erickson as Dave Mannering
Sara Lane as Kit Austin
Andi Garrett as Libby Mannering
Sharyl Locke as Tess Mannering
Patricia Breslin as Ellie Mannering
John Archer as John Austin, Kit's father
John Crawford as State Trooper
Joyce Meadows as Judith Marak
Tom Hatten as Gerald Nyes
Howard Thompson called it a "generally broad and belabored expansion of a nifty idea"; he considered redundant the "middle chapter" of the film, "involving the aroused, snarling killer" and thought the film should have "held to the impressionable viewpoint of the youngsters."
Saturday Review noted, "Unfortunately, there is little for the eye, ear, or mind in [the film]... The call, from teen-age pranksters [Lane and Garrett] probably seemed like a good "gimmick" on which to base an entire film. It isn't."
Variety commented, "[The film] is a well-produced, well-acted entry in the suspense-terror field. ... [Crawford's] slightest gesture or expression ... conveys vivid emotion."
I Saw What You Did was released on VHS and Region 1 DVD on August 24, 1999 by Anchor Bay Entertainment; both releases are long out of print. On May 23, 2014, it was re-released as an exclusive DVD-R by Universal as part of its Universal Vault Series, but in a full frame presentation.
On May 17, 2016, Shout! Factory released the film for the first time on Blu-ray Disc under their sub-label, Scream Factory.
I Saw What You Did was remade for television in 1988 with Robert Carradine, David Carradine, Tammy Lauren and Shawnee Smith.