In Milan, Donatella Berzaghi is a young woman of twenty-five suffering from a form of intellectual disability that makes her particularly vulnerable to men. She lives with her widowed father, Amanzio, who protects and watches over her to prevent dangerous situations. However, Donatella has been missing for more than a month, and Amanzio turns to the Questura to complain about the ineffectiveness of the local police station's investigations and ask for more commitment from the detectives. Commissioner Duca Lamberti listens to his appeal.
On the day of Donatella's disappearance, Amanzio had called his daughter on the phone as usual, but had received no answer. He had rushed home and found the flat empty, but perfectly in order, with locks and windows closed. The only item missing seemed to be Donatella's favourite teddy bear. Duke Lamberti suspects that the girl was taken away by someone he knew and, struck by Amanzio's suffering, decides to help him.
Lamberti starts investigating the prostitution ring, looking for clues about Donatella's past. During the investigation, he meets Salvatore, a former pimp now involved in the car trade. The commissioner asks Salvatore for information about the prostitution ring, and although the man is reluctant to cooperate, Lamberti convinces him by falsely accusing him of drug trafficking.
Salvatore introduces the commissioner to the world of prostitution by presenting him as a customer looking for a good time. The investigation continues discreetly, but Lamberti's collaborator Mascaranti threatens to ruin their cover during an operation. During a meeting with Herrero, a black prostitute, Duke Lamberti obtains some information about Donatella. The prostitute reveals the name of a client, engineer Salvarsanti, who had mentioned meeting Donatella. The commissioner manages to obtain the address of the dating house where Salvarsanti had met Donatella. However, when Lamberti and Mascaranti arrive at the site, they discover that the building has been demolished in the meantime.
One Sunday morning, Salvatore and Mascaranti bring news to the commissioner. Salvatore claims to have been contacted by people involved in prostitution, who, fearing being hunted down, are willing to reveal information about Donatella in exchange for a ransom of half a million lire. The place for the exchange is set in the car park of the San Siro stadium, but the pimps do not turn up.
Later, a hunter finds a partially charred corpse in a field: it is Donatella's body. When Lamberti and Mascaranti tell Amanzio the terrible news, the man falls ill, but the commissioner manages to revive him. At the morgue, Amanzio insists on seeing his daughter's body, and finally, resigned to her death, decides to dispose of all of Donatella's belongings. Talking to the rubbish men in charge of collecting the rubbish, Amanzio discovers that one of them has found Donatella's favourite teddy bear. The garbage man explains that he had found the teddy bear in the now demolished building about a month earlier.
Meanwhile, Salvatore, feeling threatened, tries to escape but is killed. Lamberti, furious over Donatella's death, throws Herrero, the former prostitute, out of his house, holding her responsible for the lack of information that could have saved the girl.
Amanzio, convinced that his daughter's murderer may live in the same building as them, decides to investigate on his own while Lamberti and Mascar