On a spring day in Venice, Commissario Brunetti and his assistant Vianello play hooky from the Questura in order to help Vianello's friend Marco Ribetti, arrested during an environmental protest. They secure his release, only to be faced by the fury of the man's father-in-law, Giovanni De Cal, who has made violent threats against Ribetti. Brunetti's curiosity is piqued, and he finds himself drawn to Murano to investigate. Is De Cal the type of man to carry out his threats? When the body of De Cal's night watchman is found in front of a blazing furnace, he wonders: Could the old man have killed him?