FOUR FLIES ON GREY VELVET

Rock drummer Roberto Tobias has been stalked for days by a suspicious man wearing sunglasses. One night, he chases and confronts the stalker in an empty opera house, whereupon the stalker pulls a knife. In the ensuing struggle, Roberto accidentally stabs and seemingly kills the man with his own knife. A masked figure on a balcony snaps several photographs of the incident. In the following days, Roberto receives threatening letters, photographs of the incident and the dead man's identity card, which reveals his name to be Carlo Marosi. He is also plagued by a recurring nightmare in which he is publicly beheaded in a square in Saudi Arabia, believing it is a premonition.

At home one night, Roberto is ambushed by the masked individual, who tells him they are not finished with him before knocking him out. When Roberto's wife Nina wakes up, he confesses everything to her but refuses to go to the police for fear of being arrested for Marosi's murder; the couple's live-in maid, Amelia, overhears their conversation. Knowing the masked figure's identity, Amelia tries to blackmail them and arranges a meeting with them in a park, where she is killed by the unseen assailant. Despite Roberto's reluctance, Nina's cousin Dalia arrives to stay with Nina and Roberto.

It is revealed that Marosi is still alive and conspired with Roberto's tormentor to fake his own death using a prop knife. Unnerved by Amelia's murder, Marosi tries to back out of their agreement, but the unseen tormentor kills him.

Roberto hires Gianni Arrosio, a flamboyant private investigator recommended by his beatnik artist friend Godfrey, to identify his tormentor. When Roberto returns home, he finds Nina leaving; she tells him she refuses to stay in the house while the stalker is at large. Roberto resolves to remain. That evening, Dalia offers to take care of Roberto, and the two begin an affair.

Arrosio visits Villa Rapidi, a psychiatric facility, and speaks with a doctor about an unnamed patient diagnosed as a homicidal maniac. When this patient's father suddenly died, the psychotic symptoms inexplicably disappeared, and the patient was discharged. Later, while trailing the unidentified patient in a subway station, Arrosio is ambushed and murdered with a poison-filled syringe in a bathroom stall.

Just as Dalia begins to suspect the killer's identity, the killer appears and stabs her to death while she is alone in Roberto's house. The police use optography to generate the last image Dalia saw before death but only obtain a blurry picture of four dark smudges against a grey background. The technician notes that the resulting image resembles four flies.

One night, Godfrey advises Roberto to wait in his house with a gun, deducing that the killer will come for him next. As Roberto waits in the dark, Godfrey calls to check on him, but the call is cut off. Shortly afterwards, Nina arrives home. Roberto urges her to leave for her safety, but he notices Nina's pendant necklace; it is a fly enclosed in glass, and as it swings, it gives the appearance of several flies.

As Roberto realizes Nina is the killer, she grabs his gun and shoots him in the shoulder. Holding Roberto at gunpoint, Nina explains that because her abusive stepfather wanted a son at all costs, he forced her to wear boys' clothing and ultimately committed her to the Villa Rapidi asylum. His death cured her condition but robbed her of the chance to enact revenge, leaving her frustrated. After Nina met Roberto, who bore a striking resemblance to her stepfather, she plotted to torment and murder Roberto as a proxy for her stepfather.

Nina repeatedly shoots Roberto until Godfrey arrives, distracting Nina before Roberto knocks the gun out of her hands. As Nina flees, Roberto tells Godfrey that he has realized that the protagonist of his nightmares was actually Nina. Nina speeds away in Roberto's car but crashes into the back of a truck. The truck's rear bumper decapitates her, and the car explodes into flames.

Roberto's masked tormentor is revealed after a deadly confrontation involving clues from a blurry image resembling four flies; the killer is identified and stopped.

The ending delivers Argento’s classic twist: recurring imagery and psychological motifs become central to unmasking the killer. The optography image—four flies—serves as a literal and symbolic clue, reflecting the film’s fixation on perception and identity. Justice is achieved through a mix of forensic ingenuity and poetic irony, resolving the web of paranoia and violence.