Set in Turin and starring Marcello Mastroianni, Jacqueline Bisset, and Jean-Louis Trintignant, the plot tells the murders of two ordinary individuals who are in touch with the city's élite.

Inspector Santamaria, Chief of Homicide Squad, is assigned to investigate the murder of the architect Garrone, a second-rate fellow living on the fringes of polite society, who has been battered to death with a stone phallus.

The servants of Anna Carla Dosio, the bored wife of an often away businessman, take to the police station a discarded draft of a letter she wanted to send to her dear friend Massimo Campi; in it she says that they must rid out of Garrone.

While Campi withholds co-operation, to protect his homosexuality, Anna Carla enthusiastically assists Santamaria and promises him a secret rendezvous, beginning with lunch.

By coincidence, a Saturday morning, all the involved characters, rally, for different reasons, at the Balon, the city's flea market, where Riviera has to meet with the possible murderer, but he's killed in turn by mean of a stone pestle.