When a dying hospice patient gasps that her husband was murdered over "bad money," Commissario Brunetti softly promises he and his colleague, Claudia Griffoni, will look into what initially appears to be a private family tragedy. They discover that the man had worked in the field, collecting samples of contamination for a company that measures the cleanliness of Venice's water supply, and that he had recently died in a mysterious motorcycle accident. Piecing together the tangled threads, Brunetti comes to realize the perilous meaning in the woman's accusation and the threat it reveals to the health of the entire region. But justice in this case proves to be ambiguous, as Brunetti is reminded it can be when he reads Aeschylus's classic play The Eumenides.