I SAW WHAT YOU DID

Lisa Harris, a popular high school student who is more interested in her boyfriend Louis than getting good grades, is invited to dinner by Kim Fielding, her intelligent classmate who never breaks any rules and has to babysit her sister Julia, since her father is out of town for the night. Feeling she has nothing in common with Kim, Lisa only agrees to come over to meet her boyfriend Louis there. Bored at awaiting his arrival, she joins Kim and Julia in making prank calls. When it's her turn, Lisa calls Adrian Lancer, a man with mental problems who just murdered his girlfriend Robyn Griffin for declining his marriage proposal. Lisa decides to hang up, and later calls people, saying, "I saw what you did, and I know who you are", before hanging up.

Later, Lisa and Kim discuss Kim's love life, deciding she needs an older man who appreciates her. They decide to call Adrian again, but Lisa, afraid to seduce him, repeats the line, "I saw what you did, and I know who you are." Adrian, who was caught in the act when burying Robyn's body, does not realize it's a prank, and is determined to get rid of her. Kim, thinking he was flirting with her, calls him again later, agreeing to meet with him. She is nervous to actually meet him, but she is convinced that she should drive by his house. Meanwhile, Adrian's visiting brother Stephen starts to suspect that Adrian did something to Robyn.

Taking the car to his house, Adrian notices Kim, and opens his front door. Kim, afraid of admitting who she is, pretends that her car broke down, and that she has to call for help. When Adrian lets Kim use his phone, she pretends to call someone, but mentions she is at Adrian's, who immediately becomes suspicious, as he has not told his name to her. She starts to get afraid of him, and leaves, but forgets her purse by mistake when Stephen comes back. After she drives away, Stephen informs Adrian that he told Robyn about his mental problems. Back at home, Louis and his friends finally drop by Kim's house to pick up Lisa. Not wanting to ditch Kim, she decides not to go with him. Kim still feels hurt, though, for finding out Lisa only used her for meeting friends, and Lisa soon leaves.

Stephen finds out that Adrian killed his girlfriend, but before he can do anything about it, Adrian knocks him out. Just as he prepares to burn him with gasoline, he decides to silence Kim first. After he leaves, Stephen regains consciousness, and reports him to the police. On his way to Kim's house, a policeman pulls Adrian over, and, recognizing who he is, chases him. Adrian speeds away, and runs the car off the road. It blows up, causing the police to assume he perished in the explosion; he continues on to Kim's house. Upon confronting her, she admits she prank-called him. Interrupted by a call from Lisa informing Kim about hearing on the news that Adrian murdered his girlfriend, Kim tries to warn the police, but Adrian stops her, setting the house on fire. When Adrian attempts to kill Kim, the family dog charges into him, knocking him into the fire, allowing the girls to get outside with the help of a neighbor, Randy. After Kim and the police arrive, they witness Adrian running outside, burning alive, and dying before he can hurt anybody else.

One night, after the incident is over, Kim receives a phone call from Stephen, who says, "Kim, I know who you are. You killed my brother", causing her to scream as the film ends.

Adrian is pursued by the police after being identified as the murderer, crashes his car, and is presumed dead; he continues on to confront Kim, setting her house on fire and attempting to kill her.

The climax resolves the threat posed by Adrian, whose relentless pursuit is fueled by paranoia over the prank call. The mistaken assumption by authorities that he perished in a car explosion adds an extra layer of suspense, as he continues to menace Kim. The ending underscores the unintended consequences of reckless actions and the thin line between safety and danger in seemingly mundane settings.