A young bride is murdered on her wedding day by the man she rejected for her current fiancé Len Gamble, a police detective. Several years later, a bride-to-be is stabbed to death in a movie theater with a dagger on Long Island while her friend sits beside her. The killer, Ray Carlton, flees into the night.
The next morning, the killer arrives by bus at Staten Island, where he sees from a distance university student Amy Jensen (Caitlin O'Heaney). Amy is preparing for her wedding, and she sees off her husband-to-be Phil and his two friends who leave town for a bachelor party the weekend before the wedding. After attending a ballet class with her friends Nancy and Joyce, the three run into their psychology professor Carl (James Rebhorn), 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. She is startled by Marvin (Don Scardino), her ex-boyfriend, outside the ice cream shop on a break from his job at the local morgue. She then goes to the dress shop for her fitting, and as she leaves, the dressmaker is stabbed to death with scissors by the man who was following her. Later that night, Nancy and Joyce surprise Amy at her home with a small bachelorette party. Her parents are gone for the weekend, leaving Amy in charge of her kid sister. Joyce leaves the party for Carl's house, where the two begin to have sex until the power inexplicably goes out. Carl goes to check on the electrical box, and when he returns he is stabbed to death by the killer with a chef's 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 her sister, but he declines as he has a shift at the morgue that night. Meanwhile, police find the dressmaker's body at the shop, and detectives Frank Daley (Paul Gleason) and Len Gamble arrive to investigate. Later, Amy and Nancy meet Elliot (Tom Hanks), a psychology student, while jogging through a forest trail. They later 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 Carlton 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. Nancy takes a shower and then puts on a record and lies down in the living room where she smokes a joint, and moments later has her throat cut by Ray with his chef's knife.
Amy returns, discovers Nancy's severed head in the fish tank, and is attacked by Ray. She flees outside 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 confronts Amy and chases her through a tunnel system in the morgue's basement. When confronted by Detective Gamble, the killer stabs him in the heart after he gets shot in his right shoulder by the detective. Neverless, Ray continues to pursue Amy. Amy manages to trap the wounded killer inside a storage closet and escapes from the basement where Marvin finds her. The two flee outside to safety where police are arriving and entering the morgue. Later, Marvin and Amy are to be married, as it is implied 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 camera, and says "Phil, what are you doing here?" before she screams and the screen fades to red.Don Scardino as Marvin
Caitlin O'Heaney as Amy Jensen
Elizabeth Kemp as Nancy
Tom Rolfing as Ray Carlton
Lewis Arlt as Det. Len Gamble
Patsy Pease as Joyce
James Rebhorn as Prof. Carl Mason
Dana Barron as Diane Jensen
Tom Hanks as Elliot
Paul Gleason as Det. Frank Daley
James Carroll as Phil
Russell Todd as Don
The film was originally slated to be shot in Houston, Texas, under executive producer Samuel Z. Arkoff (who had executive-produced other releases from MGM, including The Amityville Horror the previous year), but Arkoff was unable to finance the film, so as a result, it was shot entirely in Staten Island, New York, with half of the original budget. The film was shot on 35mm over a period of fifteen days in December 1979, with the film's climax being shot at the historic Seaview Hospital in Staten Island. According to director Mastroianni, the entire production from script to final edit took only six months to complete.
The original music score was composed by Alexander and Mark Peskanov. The film marked the first movie appearance of actor Tom Hanks, who played a relatively small part. In fact, it was said that Hanks' character was originally written to be killed off with Nancy's character, but because the filmmakers liked him so much, they omitted filming his death scene for the film.
He Knows You're Alone had its world premiere in Los Angeles, California, on August 29, 1980. The film opened in New York City the following month on September 26, showing at several cinemas in Manhattan.
The film received mixed to negative reviews, including one by Tom Buckley of The New York Times, citing "uncertain pacing, halting performances and innumerable technical flaws", while praising the performance of male lead Don Scardino.
Jimmy Summers of BoxOffice magazine gave the film a negative review, noting: "He Knows You're Alone is another one of those low-budget thrillers that should carry in the credits line: "Based on characters and ideas developed by John Carpenter."" Additionally, Summers noted the lack of on-screen violence as leaving the "more blood-thirsty horror fans feeling cheated.
In a retrospective, Rovi noted the film's opening sequence in the movie theater to be an almost "shot-by-shot" influence on the opening scene of Wes Craven's Scream 2 (1997).