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.