When two mischievous teens Libby and Kit are home alone with Libby's younger sister Tess, 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, a man who recently murdered his wife, Judith 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 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 the car registration of Libby's mother from the car seat before Libby drives away, and gives it to Marak, telling him to keep it as a souvenir, his last "Suzette". 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. The identification of Libby's mother 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.
Advertisements for the movie read "William Castle warns you: This is a motion picture about UXORICIDE!" and, in an early trailer for the film, Castle advised the audience that a section of the theater would be installed with seat belts for audience members "who might be scared out of their seats". The advertised gimmick was abandoned before the release of the film and never actually used. The trailer of the film has the announcer saying repeatedly: "DON'T ANSWER IT!!!".
Critical reviews
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 William Castle's egregiously low-budgeted I Saw What You Did, an attempt at terror starring Joan Crawford and John Ireland." Variety wrote " is a well-produced, well-acted entry in the suspense-terror field.... slightest gesture or expression...conveys vivid emotion."
I Saw What You Did was released by Anchor Bay Entertainment on VHS and Region 1 DVD on August 24, 1999; both releases are 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 under its sub-label Scream Factory.
