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