[2] The rude Commissioner Belli (Franco Nero) is entrusted with the investigation into the death of a record producer, this Mr. Romanis (Marino Masé).

In a whirlwind of events, the commissioner comes into contact with a series of characters: from the model Sandy (Delia Boccardo) to the pop singer Emanuelle (Susanna Martinková), from the lawyer Fontana (Adolfo Celi) to his beautiful wife, Mrs. Vera ( Florinda Bolkan).

After the death of Mino (Maurizio Bonuglia) (son of the lawyer Fontana) and Sandy, Commissioner Belli will find himself faced with the truth, unmasking the unsuspected murderess.

[2] Detective Belli was released in Italy by Interfilm on September 6, 1969, where it grossed 616.63 million Italian lira.

[2] Critic Giovanni Buttafava praised the film's ending, describing it as "nocturnal, stylized, à la Jean-Pierre Melville, with the cold yet emotionally charged confrontation between the half-rotten-half-clean cop Franco Nero and Florinda Bolkan, with stylistic traits of film noir mythologizing which were rare in Italy, and rather remarkable as well".