Maria Zani (Barbara Bouchet) is interrupted during a massage by her angry husband, Paolo (Silvano Tranquilli). He has proof she has been unfaithful to him, though she denies it. That night, someone dressed in black and wearing surgical gloves brutally murders her after injecting her with a chemical that leaves her paralyzed but still conscious. The next day, the police inspector assigned to the case, Tellini (Giancarlo Giannini), questions Paolo. The police find a picture of Maria being caressed by the hand of a man, but because half the picture is missing his identity is unknown. At home that night, Tellini confesses to his artist wife Anna (Stefania Sandrelli)) that he doesn't feel cut out for homicide investigations.Tellini continues to investigate the crime, and trace the needles used in the crime to a local doctor. The doctor protests his innocence, and when Tellini leaves the office he is accosted by Paolo. Paolo too insists he is innocent and plans to conduct his own investigation. Meanwhile, the killer strikes again, this time murdering a clothing store owner with no connection to Maria.Tellini visits a scientist acquaintance of the woman, who demonstrates that a species of wasp will use a toxin to paralyze and eviscerate a tarantula in order to lay its eggs in the corpse. Tellini has the scientist arrested on drug possession charges.Laura (Claudine Auger), who owns the spa that Maria patronized the day of her death, phones Mario (Giancarlo Prete), who was Maria's lover in the picture. He and Laura take photos of lovers in order to blackmail them, and she tells him to deliver the last batch of photos of Maria's indiscretions to a woman named Franca. When Mario goes to deliver the package, Tellini and Paolo (who is now working with Tellini) chase him. Paolo falls to his death after a brief struggle with Mario, but Mario is then run down by a car in the street below.Shortly after being interviewed by Tellini, Franca is murdered by the killer. But the police rule that Mario was the killer of all three women. When a tape of Tellini and his wife making love is made public, Tellini again determines to leave the force. But when he is nearly killed in a staged automobile accident, he realizes he needs to solve at least this one last crime.Jenny (Barbara Bach), one of Laura's spa employees, resigns in protest of the blackmail ring. Laura obliquely threatens her life, but their conversation is interrupted by the spa's blind masseur (Ezio Marano). Jenny spends the night at a friend's house, but is followed there by the killer who murders her then leaves her body in a trash bag to be found the next morning. Tellini interviews some of Jenny's co-workers, including the aloof Laura, a nurse who wears gloves identical to the killer's and the blind masseur, who takes off his darkened glasses to reveal colorless, unseeing eyes.That night, Laura telephones Tellini to inform him that she has determined the killer's identity. But when he goes to the spa, he finds her dead with a colorless contact lens next to her body. Realizing the masseur had been faking his blindness and was indeed the killer, Tellini races home to find the killer attacking Anna. The men struggle, and Tellini subdues him and saves Anna.The next day, a psychiatrist tells Tellini that the masseur had begun faking his slightlessness after killing his unfaithful, sexually voracious wife; he then continued to kill other women in order to satisfy his inner demons. Satisfied at solving the case but still disillusioned with police work, Tellini wanders the crowded streets of Rome.