A dying patient at the hospice Fatebenefratelli wants to speak to the police, and Commissario Guido Brunetti and his colleague, Claudia Griffoni, waste no time in responding. Benedetta Toso believes that her recently deceased husband, Vittorio Fadalto was murdered. Looking into the apparent private family tragedy, they discover that Fadalto worked in the field collecting samples of contamination for a company that measures the cleanliness of Venice's water supply-- and that he had died in a mysterious motorcycle accident. Brunetti comes to realize the perilous meaning in the woman's accusation and the threat it reveals to the health of the entire region.