Inspector Montalbano discovers the existence of a secret underground dwelling beneath Mimi's family home and finds a trunk containing the dead body of a young woman who had disappeared six years earlier. As he begins his investigation, Montalbano makes the acquaintance of the victim's alluring twin sister.

The naked body of a young woman is discovered on a beach near Vigata. The girl was shot in the face, her identity is unknown and the only clue police have to go on is a butterfly tattoo on her shoulder. Montalbano sets to work trying to establish the girl's identity and investigates possible links between the murder victim and a local charity with connections to the church. Meanwhile, his relationship with Livia continues to suffer as a result of his poor work/life balance.

Montalbano wakes up one morning to find the carcass of a dead horse stranded on the beach below his house. Soon after, a beautiful woman walks into Vigata police station to report the abduction of her racehorse. A line of investigation emerges that brings Montalbano into contact with the elegant world of horse racing, via some illegal horse betting rackets organised by the Mafia.

A distressed woman arrives at Vigata police station to report the disappearance of her brother. Soon enough, the man is found murdered in what appears to have been a crime of passion. But Montalbano is reluctant to give too much credit to appearances and digs into the victim's past, talking to anyone who might have had an involvement in the case. Meanwhile, a number of high-profile businessmen and politicians die as a result of cocaine overdoses.