I Saw What You Did (1965)

Mid-teen friends Libby Mannering and Kit Austin, along with Libby's young adolescent sister Tess Mannering, are spending the evening together at the Mannerings' isolated country home without any adult supervision, Mr. and Mrs. Mannering who trust Libby to babysit Tess for the first time in keeping to their out of town overnight plans even after their original babysitter had to cancel at the last minute. Struggling for things to do in their isolation, the three decide to do something Libby and Tess do occasionally, which is to make prank telephone calls to people they don't know chosen randomly out of the telephone book. While the nature of the initial calls are all over the map, they eventually settle on a set script spoken seductively: "I saw what you did and I know who you are." While they receive a variety of responses from the people they call, they are especially intrigued by that of Steve Marak, who impressionable Libby, the caller in this instance, falls for by voice and response alone. What none of Libby, Kit and Tess are aware of is that the reason for Steve's interest in the call is that he has just murdered and buried his wife, Judith Marak, in their continuing troubles, which Judith may or may not have known includes infidelity on Steve's part with their neighbor, Amy Nelson. With the call, Steve is afraid that the caller saw him murder and/or bury Judith. The call begins Kit and Steve's mutual obsession with each other, but for different reasons, Steve's which is to discover who the caller is and how to find her to dispose of her. Factoring into what happens is controlling Amy, who is obsessed with Steve herself, he in return having no intention of doing what she wants which is to make her the next Mrs. Marak.