Roberto Tobias, Michael Brandon, a drummer in a rock band, has been stalked for days by a suspicious man in sunglasses. One night, the chase leads him into an empty opera house, where the stalker closes in with a knife. In the struggle that follows, Roberto accidentally stabs the man with his own knife, and the assailant collapses. A masked figure on a balcony snaps several photographs of the incident. In the days that follow, Roberto is bombarded with threatening letters, pictures from the scene, and the dead man’s identity card, which reveals the name Carlo Marosi. He is also haunted by a recurring nightmare in which he is publicly beheaded in a square somewhere in Saudi Arabia, a vision he believes might be a premonition.
Back at home, Roberto is suddenly ambushed by the masked attacker, who warns that their work isn’t finished before knocking him unconscious. When Nina Tobias, Mimsy Farmer, wakes, Roberto confesses everything but refuses to go to the police for fear of being arrested for Marosi’s murder; the couple’s live-in maid, Amelia, overhears the confession. Amelia, who tries to blackmail them with her knowledge of the killer’s identity, arranges a park meeting with the assailant, only to be killed by the unseen foe. Despite Roberto’s reluctance, Nina’s cousin Dalia, Francine Racette, arrives to stay with them.
It’s soon revealed that Marosi is very much alive and has been conspiring with Roberto’s tormentor to fake his own death with a prop knife. After Amelia’s murder, Marosi tries to back out of the plan, but the unseen tormentor cuts him down anyway.
To uncover the truth, Roberto hires Gianni Arrosio, Jean-Pierre Marielle, a flamboyant private investigator recommended by his beatnik artist friend, Godfrey, Bud Spencer. Arrosio’s investigation leads him to Villa Rapidi, a psychiatric facility, where a doctor explains the case of an unnamed patient diagnosed as a homicidal maniac. When this patient’s father dies, the psychotic symptoms mysteriously disappear and the patient is discharged. Later, while trailing the unnamed patient through a subway station, Arrosio is ambushed and murdered with a poison-filled syringe in a bathroom stall.
As Dalia begins to sense who the killer might be, the murderer finally appears and stabs her to death while she’s alone in Roberto’s house. The authorities attempt to reconstruct the last image Dalia Saw using optography, but the result is a blurry photograph of four dark smudges against a grey background, described by the technician as looking like four flies.
One night, Godfrey advises Roberto to wait inside the house with a gun, certain that the killer will come for him next. As Roberto waits in the darkness, Godfrey checks in by phone, but the line is cut off. Nina returns home, and Roberto urges her to leave for safety. He notices Nina’s pendant—a glass-encased fly—that swings and briefly resembles several flies, a clue that resonates with his nightmare imagery.
The truth crystallizes: Nina is the killer. She confesses that her abusive stepfather, who demanded a son at any cost, forced her into boys’ clothing and eventually committed her to the Villa Rapidi asylum. His death cured her condition but deprived her of any chance to enact revenge, so she targeted Roberto, who bore a striking resemblance to her stepfather, as a proxy for her vendetta. Nina shoots Roberto repeatedly until Godfrey arrives, distracting her long enough for Roberto to disarm her.
As Nina attempts to flee in her car, Roberto tells Godfrey that the figure from his nightmares was actually Nina all along. She speeds away, crashes into the back of a truck, and is decapitated by the truck’s rear bumper as the car erupts in flames.
