A dying hospice patient gasps these cryptic words about her recently-deceased husband. Brunetti softly promises he will look into what appears to be a private family tragedy. He discovers that her husband worked for a company that monitored the cleanliness of Venice's water supply and that he had died in a motorcycle accident.

Distracted briefly by Vice Questore Patta's obsession with youth crime in Venice, Brunetti turns to the remarkable research skills of Patta's secretary, Signora Elettra Zorzi. With her help, Brunetti comes to realize the perilous meaning in the dying woman's accusation and the threat it reveals to the health of the entire region.