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