HE KNOWS YOU'RE ALONE

A young bride is murdered on her wedding day by the man she rejected for her current fiancé, Len Gamble, a detective. Several years later on Long Island, a young bride-to-be named Marie is stabbed to death in a movie theater while her friend Ruthie sits beside her. The killer, Ray Carlton, disappears into the night.

The next morning, Ray arrives on Staten Island, where he observes university student Amy Jensen from a distance. Amy is preparing for her wedding. She, her fiancé, Phil, and his friends on their way out of town prepare for a bachelor party before the wedding. After attending a ballet class with her friends, Nancy and Joyce, the three run into their philosophy professor Carl, with whom Joyce is having an affair. Amy leaves to go to a dress fitting, stopping to get ice cream on the way, where she notices a man following her. Outside the ice-cream shop, she is startled by Marvin, her ex-boyfriend, who is on a break from his job at the local morgue.

Amy stops by the local dress shop for her fitting. Unbeknownst to her, as she leaves, the dressmaker is stabbed to death by Ray with a pair of scissors. Later that night, Nancy and Joyce surprise Amy at her home with a small bachelorette party. Her parents, gone for the weekend, leave Amy in charge of her kid sister, Diane. Joyce leaves the party for Carl's house, where the two begin to have sex, until the power inexplicably goes out. Carl checks on the electrical box. When he returns, he is stabbed to death by the killer with a kitchen knife after finding Joyce's lifeless body in the bed.

The following morning, Marvin arrives at Amy's house and insinuates that he wants to rekindle their relationship, and Amy expresses second thoughts over her marriage to Phil. While in the kitchen, Amy sees the mysterious man standing in her yard and becomes frightened. She invites Marvin to come to a local amusement park with her, Nancy and Diane, but he declines because he has a shift at the morgue that night.

Meanwhile, the police find the dressmaker's body at the shop. Detectives Frank Daley and Len Gamble arrive to investigate. Later, Amy and Nancy meet a student named Elliot while jogging through a forest trail. They attend the amusement park with him, where he questions Amy's claims of a man following her. While riding a dark ride with her sister, Amy sees Ray inside the ride, and confides in Nancy at her house that night. Amy briefly leaves to take her sister to a birthday party, leaving Nancy alone at the house. After taking a shower, Nancy puts on a record and lies down in the living room to smoke a joint. Moments later, her throat is slashed by Ray.

Amy returns and is attacked by Ray after discovering Nancy's severed head in the fish tank. She rushes to her car and struggles to drive with Ray on the roof. She crashes the car in a wooded area and runs to the nearby morgue, where she finds Marvin and phones the police. Ray enters the morgue, and Detective Gamble arrives as well. Ray chases Amy through a tunnel system in the morgue's basement. When confronted by Detective Gamble, Ray stabs him in the heart after he gets shot in his left shoulder. Nevertheless, Ray continues to pursue Amy. Amy manages to trap the wounded Ray inside a storage closet and escapes from the basement with Marvin. The two flee outside as the police arrive and enter the morgue.

Marvin and Amy are to be married, implying that she cut off her marriage to Phil. As Amy sits in front of a mirror in her wedding dress, an unseen person enters the room. She stands, approaches the individual and says, "Phil, what are you doing here?", before screaming in horror.

Amy is attacked by Ray after discovering her friend's severed head. She escapes, crashes her car, and flees to the morgue where the final confrontation with the killer takes place.

The ending delivers the slasher genre's expected confrontation between final girl and killer, emphasizing Amy's resourcefulness under duress. The morgue setting intensifies the horror and brings the narrative full circle, with the killer's obsession and the protagonist's survival instinct colliding in a chilling, claustrophobic environment.