- Teenagers Libby and Kit innocently spend an evening making random prank calls that lead to murderous consequences.
- 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.—Huggo
- When two teenagers make prank phone calls to strangers, they become the target for terror when they whisper "I Saw What You Did, And I Know Who You Are!" to psychopath Steve Marek (John Ireland) who has just murdered his wife (Joyce Meadows). But somebody else knows of the terrible crime that was committed that night, the killer's desperately amorous neighbor Amy Nelson (Joan Crawford).—Joan Crawford & William Castle
- Two teenage girls occupy their time by randomly calling strangers, whispering "I saw what you did," and hanging up. When one of their victims turns out to be a man who has just murdered his wife, he thinks they witnessed the crime and sets out to find the girls and kill them.—Kelly
- Mr. and Mrs. Mannering are about to leave on an overnight business trip when the babysitter cancels. They reluctantly leave sixteen-year-old Libby and her younger sister Tess home alone. Libby invites her friend Kit Austin to spend the night. Kit's father allows her to come for dinner but not to stay all night, since the parents are not home.
The Mannerings live on an isolated property several miles from town. Kit is given a tour, ending in the barn where the door blows shut. The girls are unable to open it and Tess, who has followed them, lets them out. She plays a prank on Kit by telling her that a boy is on the phone wanting to speak to her. When Kit picks up the receiver, no one is there. Libby scolds Tess but admits that they have fun making prank phone calls. Kit is mystified so the Mannering girls show her how it's done. They choose a number at random from the phone directory. In a sultry voice, Libby says "I saw what you did and I know who you are." Kit thinks this is hilarious. They call several people and get various responses, from indignation to amusement.
Meanwhile, Mrs. Mannering is repeatedly calling home to check on her daughters but gets a busy signal. Her husband assures her that he'd be worried if they weren't on the phone.
When the girls dial Steve Marak's number, his wife Judith answers the phone. Libby asks for Steve and Judith goes to get him. An open suitcase is on the bed and water is running in the bathroom. Judith tells Libby that Steve is taking a bath. Libby purrs that she really needs to speak with him. When Judith goes into the bathroom, she sees that Steve has slashed her clothing with a knife. She begins screaming at him. He yanks her into the shower and stabs her to death, then throws her body through the glass door.
Since no one comes to the phone, the girls eventually hang up and decide to take a break.
Steve's next-door neighbor Amy lets herself into the house. She sees the suitcase on the bed and knocks on the bathroom door. Wearing a bathrobe, Steve emerges and tells her that Judith has left him. He says they had a terrible fight and she left without bothering to pack her things. Amy is happy to hear this and says Judith was too young and flighty for him. She begins packing Judith's clothing and Steve tells her to leave it alone. When she offers to straighten up the bathroom, he yanks her away and walks her into the living room. He is trying to act normally but all he wants is for her to leave. She agrees to come back later and notices that the phone is off the hook. After she leaves, he replaces the receiver and goes into the bedroom.
The girls, giggling hysterically, resume calling people. They wonder if the man is out of the bathtub yet but can't remember his name. Tess finds it because she put her peanut butter and jelly sandwich on top of it. Libby calls and this time Steve answers. He panics when a voice tells him she saw what he did and she knows who he is. Libby plays along when he asks for her name and tells him she is Suzette. He demands to meet with her and she hangs up, suddenly bored with the game.
Amy, watching from her window, sees Steve putting a large trunk into his station wagon. When he drives away, she goes over and checks out the bathroom. She puts two and two together when she sees the shattered shower door and a bloody knife in the clothes hamper.
Steve drives to a wooded area and digs a deep hole for the trunk. He is startled when a dog appears and begins barking at him. It belongs to a young couple who stopped so the dog could do its business. The woman catches a glimpse of Steve through the trees and quickly takes the dog back to the car. She tells her husband that the next time, he can chase the dog. They drive away but she is uneasy about what she saw.
When Steve returns home, Amy comes over. He is beginning to feel stifled by her attention, saying it is late and he is going to bed. As she opens the door, the phone rings. He goes to answer it and Amy slips into the bedroom to listen on the extension. Libby, pretending to be Suzette, plays a cat and mouse game with Steve. He again wants to meet with her but she hangs up, this time intrigued and wondering what he looks like.
Amy angrily asks who Suzette is and Steve tells her it is a business deal. He says he may have to go out later. By sweet-talking her and telling her he loves her, he persuades her to go home. The girls speculate about Steve and wonder how old he is. Kit asks Libby if she would actually meet him somewhere and what would she do if he tried to get romantic. Libby admits that she might let him kiss her. She envisions Steve as a handsome man and wishes they had a way to see what he looks like. Since she has recently gotten her driver's license, she suggests they drive to Steve's house in her mother's car to try and catch a glimpse of him. Kit doesn't want to but lets herself be talked into it. Libby changes into a dress and puts on a lot of makeup, trying to make herself look older.
On the way to the Marak home, Libby orders that no one is to get out of the car. When they arrive, the house is dark except for a light in the back. Kit wants to leave but Libby exits the car to peer through the windows. Steve sees her and goes into the kitchen for a knife. He is at the door when Amy appears, having seen Libby from her window. She yanks Libby's hair and drags her away. Kit and Tess immediately hide in the back seat. Calling her a tramp, Amy tells her to clear out. Libby tries to explain but Amy won't listen. Rather sadly she tells Libby that she is far too young for Steve. Then she opens the car door and shoves Libby inside. Seeing the car registration pasted on the steering post, Amy tears it off. Libby, shaking with fright, drives away.
Amy confronts Steve and tells him there will be no more Suzettes. She points out that Suzette even lied about her name, showing him the registration card. Steve tells her she doesn't own him yet. Then Amy tells him she knows he killed Judith. Unless he agrees to marry her right away, she will go to the police. "Life with me or no life at all," she says. Steve has no choice but to accept. Amy makes them drinks and reminds him of how wealthy she is; that they can go anywhere in the world. Steve goes into the kitchen for ice cubes and retrieves his knife. He kills Amy and takes the registration card, which has Mrs. Mannering's name and address. Climbing into his car, he sets off.
Libby is frantic about losing the registration card because her parents will find out that she took the car without permission. She thinks Amy was Mrs. Marak and that maybe if she returns in the morning, she can get the card back. Kit is fed up with Libby and tells her to forget about the card. Tess is supposed to be in bed, but the girls discover her missing. They go outside to find her and this time all three end up shut in the barn.
Mrs. Mannering has continued to call home and even had the operator check the line. Now there is no answer. She insists that her husband call the police to check on the girls. They arrive at the same time as Kit's father, who has come to collect his daughter. Together they search the property and find the girls in the barn. Libby calls her mother and assures her that everything is fine. Everyone leaves and the Mannering girls go to bed.
On the way home, Kit hears a breaking news story on the radio. The body of a young, attractive woman was discovered buried in a trunk not far away. A couple who were walking their dog reported seeing a middle-aged man, tall and dark-haired, in the vicinity. Kit shudders and turns off the radio. Her father knows something is wrong and says she may as well level with him. Kit says nothing but starts to wonder.
Tess is awakened by their dog barking. Realizing he has somehow gotten out she wakes Libby, who tells her to let him in. When she goes outside, Steve is waiting. He asks for Suzette and Tess, too little to realize the danger, invites him in. She wakes Libby and tells her that Steve is downstairs. Libby is startled but goes to meet him, hoping he has come to return the registration card. She apologizes profusely for upsetting "Mrs. Marak" and says they were just making prank phone calls. Steve is relieved when he realizes she knows nothing of the murders. He scolds her and gives her back the card. As she closes the door behind him, the phone rings. Instead of leaving, Steve goes to the window to listen.
Kit is calling to tell Libby about the news report. Libby says the man's description sounds a lot like Steve and that he has just left after returning the registration card. Kit abruptly hangs up after her father asks who she is talking to so late. Libby hesitates, then dials the operator and asks for the police. Steve crashes through the window and she runs upstairs, screaming at Tess to lock the door. But Tess is not in her room. Libby realizes she must have gone outside and slips out to look for her. Steve goes after them. Libby finds Tess and they run toward the car. Someone grabs Tess and claps a hand over her mouth. Libby is unable to start the car and then Steve, who was hidden in the back seat, tries to strangle her. A shot rings out and he collapses. Mr. Austin and the police are there. He grabbed Tess to prevent her from crying out. The girls embrace, sobbing. Mr. Austin says it is a good thing his daughter finally told him the truth. Libby tearfully says they will not be using the phone again for a long, long time.
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