Commissario Guido Brunetti is kicking his heels, pondering the recent lack of crime in Venice, when a beautiful young woman appears at his office door.

Now calling herself Maria Testa, his visitor is more familiar to Brunetti as Suor'lmmacolata, the nun who once cared for his mother at the casa di cura in Dolo. But Maria has recently left her convent after the unexpected deaths of five patients.

Brunetti offers to make some enquiries, but finds now obvious cause for concern. Is Maria simply creating fears to justify abandoning her vocation?

Or has she stumbled on to a deeply rooted, far more sinister scenario - and put her own life in very grave danger . . . ?