In Venice, Italy, Commissario Guido Brunetti comes to the aid of a young Catholic sister, who has left her convent after five of her nursing home patients died unexpectedly. In the course of his inquiries, Brunetti encounters an unusual cast of characters, but discovers nothing that seems criminal. The police detective must determine whether the nun is simply creating a smoke screen to justify abandoning her vocation-or if she has stumbled onto something very real and very sinister that places her own life in imminent danger.