An unidentified fifteen year-old girl is found hanged after an anonymous tip-off. The girl, Silvia Polvesi, is soon discovered to have been murdered. A peeping tom is caught with photos of her having sex with a teenage drop-out but he is later released due to lack of evidence. The investigation, conducted by Inspector Silvestri and the assistant public prosecutor Vittoria Stori, focuses on the girl's parents. It transpires that a private detective, Ruggero Pollente, was hired by Silvia's mother because the latter was concerned about her activities. As a conspiracy begins to emerge, Pollente's body is found dismembered and his girlfriend is subsequently attacked by a machete-wielding killer clad in motorcycle gear...