A group of police officers burst into a room where a nude female body is hanging from the ceiling. District Attorney Vittoria Stori arrives where she observes the scene to be that of a suicide, with the door bolted from the inside. The following day, Inspector Valentini gives Vittoria the autopsy report, saying that the girl, Sylvia, was 15 and had sexual relations before her death. Valentini talks to Sylvia's maid (her parents are in Africa), who says that Sylvia was being frightened by daily phone calls in which the caller was not responding. Vittoria watches footage of a demonstration and sees Sylvia walk into a building around the time of her death. Sylvia was found in another building, so Vittoria surmises that was murdered.
Homicide Inspector Silvestri takes over the case. While visiting the crime scene, a man is spotted photographing from the room of a nearby building. The man, Mr. Paglia, is questioned; he has many pictures of Sylva having sex with a man, taken over a three week period. Silvestri talks to Marcello, the man from the photographs, and he has an alibi (he was been spelunking for days). Marcello tells the inspector that he was not Sylvia's first or only lover. Sylvia's parents return from their African trip and ID the body.
The apartment where Sylvia was murdered is discovered; inside, there are photographs and a tape recorder, and the bathroom walls are covered in blood. Mr. Paglia is set free (the cops have nothing to charge him with). Silvestri tells reporters that they do not know if the blood in the bathroom is human.
Sylvia's mother tells Vittoria that six months ago she found birth control pills in her daughter's coat pocket, and when she confronted Sylvia about it, her daughter was indifferent. The mother threatened to tell the father, and Sylvia threatened to cut her own throat with a straight razor. Sylvia's mother hired a detective, but he later dropped the case. Silvestri goes to see Sylvia's doctor, who says she was like any other girl her age. The detective's wife does not know where her husband is, and suggests asking his secretary Rosa (his "latest lay"), in the clinic for a car accident. When the police find the detective's car, he is in the truck in many plastic wrapped pieces.
Rosa, upset about her lover's demise, refuses to answer Silvestri's question (where the detective got the money to pay for her three operations); the inspector suggests blackmail. That night a cleaver-wielding person wearing black leather and a motorcycle helmet is outside the clinic. Rosa calls Silvestri and says she now wants to help. The person enters Rosa's room and tells her to say where her lover hid everything. Rosa screams, and the person runs out, followed by Silvestri and a fellow cop. The cop gets his hand chopped off when going for a light switch. Silvestri jumps into a car with Georgina. They chase the killer (on a motorbike), but the killer drives into an old train tunnel and gets away.
Back at the clinic, Rosa shows the police the package in the vent, containing tapes. The recordings are of young men and sexual encounters with young schoolgirls, one may mentioning payment and in one instance a girl being raped. Vittoria plays some of the tapes for Sylva's mother and the maid, and they agree that it is the voice of Sylvia. The DA says there are six or seven different voices on the recording. Georgina looks for motorcycles with matching bike tracks, and learns of a butcher (who might have selected a butcher's cleaver as a weapon of choice), purchasing a bike six weeks ago. Later, the cop tells Silvestri and he talked to the butcher's father, and he said that the boy left three months ago and provided Georgina with a photograph.
The next day, Silvestri passes by Valentin and his daughter Patricia; he stops when Patricia says "good morning" (recognizing her voice from the recordings). The inspector asks Valentine to his office, and Valentine is furious that his daughter is involved with the schoolgirl prostitution ring. Vittoria tries to show Silvestri a threatening note she received the previous night, but he tells her he gets those all the time. She rips up the note. When the DA is dropped off in a parking garage, she is attacked by the killer. She runs and hides behind cars until her driver returns for something she has forgotten. The driver is killed, and Vittoria runs to the elevator. The elevator stops on the third floor and the killer crashes through the door, but runs when someone calls out. The building superintendent clams Vittoria as the killer speeds away on his motorbike.
The cops are looking for a butcher named Roberto whom is a suspect in the killings. Valentini has Patricia talk to Silvestri, but she can tell them only that a fellow student, named Julianna, told her about the prostitution. Julianna is picked up, and Silvestri asks her who Bruno is (she had said the name on the tape). She says she has only talked to him over the phone. The police learns that Bruno is the photographer, Mr. Paglia, but the killer has already paid him a visit and stabbed him in the stomach. Bruno dies in the ambulance.
The following day, Silvestrai mis-informs reporters that he survived the attack and has told the police everything. Vittoria is angry about the inspector's fabrication, but he tells her that he is trying to weed out the killer and/or people responsible for the prostitution ring.
After the story of Bruno's reputed incriminating confession, Sylvia's doctor is found dead in the bathtub from an apparent suicide. Vittoria talks to the quiet girl who attended the doctor's funeral, and she girl tells her of how, for her appointments, the doctor would drug her and do certain things to her. She says that once there was another girl there screaming, and another time a man did things to her. Silvestri returns with many files from the doctor's office, incriminating a number of men involved in the ring (the doctor was "recruiting" the girls). Two young girls telephone the police and says they saw the "fellow on TV" ride into a garage. The police surround the area. When the black-leather clad biker attempts to shoot his way out, he is gunned down and killed.
Silvestri stops a superior and shows him all the files of people trying to bring down; the superior tells him that it would take too much time, and with the killer dead and his promotion, he should not care about the prostitution ring. Silvestri is upset, but Vittoria is there with him, and Valentini tears up his resignation.