Roberto Tobias (Michael Brandon) plays the drums for a local Rome rock and roll band when at various time, he sees a man in dark sunglasses wearing a suite and tie, watching him. After the session, Roberto sees the man and follows him through the dark streets to an apparently abandoned opera house where he confronts the man and asks him why he's been following him for the past several weeks. The man declines such actions and pulls a knife on Roberto when the drummer gets too close. In the struggle, the man is accidentally stabbed and he falls from the stage to the lower level. Suddenly a spotlight is turned onto Roberto and a masked person on the balcony snaps some photos of Roberto holding the bloody knife.
That night, Roberto returns home and lies in bed awake as his wife Nina (Mimsy Farmer) lies beside him. The next day, Roberto reads the newspaper describing the dead man and he receives a letter containing the identification of a certain Carlo Marosi, the man who Roberto stabbed. That evening at a get-together of several band members and friends at his house, one of the guests talks about beheading executions in Saudi Arabia, and Roberto looks through some record albums and sees the photos of the incident. Amelia (Maria Fabbri), the maid, sees him and the photos, but does not tell him that she knows. That night, Roberto has a disturbing dream about him being beheaded in a coliseum in Saudi Arabia when he wakes up after hearing a noise. Roberto looks around and a cord is wrapped around his neck. The masked person tells Roberto, he could kill him now, but will not for he is not finished with him, and knocks him out before running away. Nina walks in and asks her husband what is wrong and he finally admits to the accidental stabbing and subsequent harassment, and says that they cannot go to the police.
Roberto goes to see Godfrey (Bud Spencer) (whom Roberto's annoys by nicknaming him 'God'). Godfrey is a beak nick artist living in a shack outside Rome with his colleague, a con-artist known only as the Professor (Oreste Lionello). Roberto confides in them about his problem and Godfrey suggests having the Professor keep an eye on him. Later, Roberto jumps out at a person who approaches his house in the pouring rain and beats him with a stick. But it is only the slow-witted mailman with a special delivery.
Meanwhile, Amelia calls someone and says that she knows what the person is doing to Roberto. She wants blackmail money or she'll go to the police. The unseen person has a flashback episode of being committed to a lunatic asylum and being tied down on a bed. Amelia goes to a local park and waits on a bench. As night falls, the park crowd dissipates, and she stands to leave when she hears a person say her name. When Amelia discovers that she is locked inside the park, the runs along the high wall and cries out for help. A couple on the other side hear her, but the man is unable to scale the stone wall. Before he can get to the gate entrance, Amelia screams and is killed by the unseen person who slashes her throat with a straight razor.
A little later, Nina arrives at a train station where she picks up her cousin Dalia (Francine Racette) and she joins Roberto's group for another get-together in the house of playing music, smoking dope, and political discussions. Roberto is the only person who does not seem to want Dalia there. Mikro, Roberto's band mate, asks why Roberto did not show up for rehearsals that day. Then Nina gets a phone call and learns that Amelia has been murdered. Roberto later has the same dream of being decapitated again and he wakes up when a noise is heard. He investigates but only hears his pet cat hissing. The next morning, there is note from the killer, and Nina is now frightened.
Meanwhile, it is revealed that Carlo Marosi is alive and well, and eating at a local restaurant. Carlo calls someone and asks them to meet at his place. referring to someone being killed. At Carlo's small apartment, he tells the unseen person that what they agreed to in harassing Roberto and mentions the "toy" (a knife with a trick blade). Carlo had been approached by the unseen killer to set this whole thing up, but now Carlo wants to back out. But the killer picks up a blunt object and hits Carlo on the head. The unseen killer gets a wire and twists around the man's neck, decapitating him. The killer then disposes of the dead Carlo to make sure he is not found. Elsewhere, the Professor tells Roberto that he saw someone last night in his back garden, with his cat wrapped in a blanket. He tried to stop the person, but got hit on the head. The Professor tells Roberto that he may seek outside help to learn who is harassing him.
Roberto goes to meet with Arrosio (Jean-Pierre Marielle), a flamboyantly gay private investigator. After the drummer tells the PI his story, Arrosio admits to never having solved a case, but is optimistic that his bad record will be broken. During a drive with Roberto, Arrosio asks him questions about his life and about Nina, when they met and how long they were married, and Roberto mentions Nina getting a big inheritance. After dropping off Arrosio at his apartment, Roberto returns to his house where Nina is leaving town with police officers about the Amelia murder. She tells Roberto that she does not want to stay in the house anymore with someone stalking them. But Roberto decides to stay and invites Dalia over to spend time with him. That evening Roberto takes a bath when Dalia walks in and admits that she has had romantic feelings for him and the two of them make love. Afterwards, Arrosio arrives and is a little surprised to see Roberto with Dalia and that Nina has left. Roberto gives Arrosio some photos of his past and his family as well as Nina's and Dalia's. Then, they find Roberto's pet cat's head and wrapped in plastic. That night, Roberto has his nightmare again about the decapitation execution, and wakes up in a cold sweat. Dalia comforts him.
Meanwhile, Arrosio is in his office looking at photos of Roberto's family and friends, as well as some old papers and financial records. Arrosio phones Roberto and tells him that he's found a "strange resemblance" in one photo, but tells him that it may only be a red herring. Arrosio tells Roberto that he's found the name of "Villa Rapidi" and will be investigating it and he asks Roberto about it, but Roberto claims to have never heard it before. The next day, Arrosio arrives at the Villa Rapidi Psychiatric Clinic where a doctor tells the private investigator about a patient that Arrosio is inquiring about (the name is not mentioned) who stayed there for three years as a teenager, whom was diagnosed as a homicidal mania. When the father who committed the teenager died from a sudden heart attack, the mental symptoms disappeared and the patient was deemed cured. The doctor also suspects that the man who committed the teen and the teen's mother there was not the patient's real father.
Arrosio talks to various people around Rome looking for the nameless ex-patient from Villa Rapidi and then visits an estate-turned-boarding-house where he talks to the landlord about the patient he is looking for. The boarding house is the residence of the killer. Arrosio follows the unseen person from the estate and onto a subway train. He gets off at the person's stop and follows the unseen person to the mens room where the unseen killer attacks him in a stall, and stabs him in the chest with a syringe of a glowing blue poison. The killer leaves, as Arrosio lies dying on the restroom floor. But with a smile on his face and with his last breath, Arrosio mumbles, "I was right...".
Roberto learns of Arrosio's murder and meets with Godfrey and the Professor at a convention hall where coffins are being sold. Roberto tells them about his nightmares and Godfrey thinks that it might be a premonition of something to come. Godfrey suggests that someone with a grudge against Roberto is trying to drive him crazy and wants him to leave Rome at once. But Roberto refuses, determined to find the killer on his own.
A few days later, Dalia calls the studio asking for Roberto, but he is busy recording music with his band. As Dalia packs her suitcase, she notices a strange similarly between a recent photo of Roberto and Nina with some unseen person in another photo. Just then, Dalia hears a noise and is frightened. Dalia slips off her shoes and sneaks up to the attic where she arms herself with a knife and waits as she hears the intruder looking for her. Dalia hides behind a door with the knife when the killer comes inside, then leaves. When Dalia thinks the killer is gone, she steps out of the doorway when a knife hits her on her forehead. Dalia stumbles down the attic stairs and is stabbed to death by the unseen killer.
After finding the body, Roberto calls the police and they tell him about a test they will do on Dalia. By removing one of her eyes and shooting a laser through it, they will be able to see the last image that Dalia had seen for the image is retained on the retina for several hours after death. On a computer screen, they see only four dark smudges against a gray background which looks like, as the technician puts it, "four flies on gray velvet." The test is declared inconclusive.
That night, Roberto loads a gun and sits in his dark home, waiting for the killer to make his move. He nods off and begins dreaming again, and his dream goes all the way with the gory beheading of a criminal in Saudi Arabia. Roberto is woken up when the phone rings and it is Godfrey asking if the drummer is okay. Roberto says that he is, and then the line goes dead. A few minutes later, Nina arrives home from her long getaway and Roberto almost shoots her as she walks through the front door. Roberto puts down the gun and tells her to leave and tries to push her out the front door, when Nina's necklace (a fly enclosed in glass) swings.... giving the appearance of more than one fly, and Roberto pulls her back inside and hits her. Roberto confronts Nina and accuses her of killing Amelia, Arrosio, and Dalia, and the one who terrorized him. Nina grabs Roberto's gun and shoots him the in shoulder. As Roberto lies wounded on the floor, Nina suddenly adopts a sneering, wild-eyed appearance. Nina mechanically laughs and she tells Roberto about her abusive stepfather who placed her in a lunatic asylum when she was little and when he died, her mental condition was cured. But when Nina apparently met Roberto many years later, he reminded her of her late father. So, Nina married Roberto and cooked up this whole event as part of her twisted way of getting back at her father, since Roberto is apparently the dead-splitting image of Nina's late father. Nina shoots Roberto a few more times in his arm and both legs, when Godfrey runs in and Roberto knocks the gun out of Nina's hands. Nina runs to Roberto's car and speeds away. But in a twist of fate, she doesn't look where she is going and rams into the back of a truck, and she is decapitated by the truck's rear bumper as it smashes, in slow-motion, through her car windshield. The car then explodes in a mass of flames.