A beautiful young woman appears on the threshold of Commissioner Brunetti's office. The Commissioner recognizes her immediately: this is Sister Immacolata, the nun who cared for his mother in the nursing home. But to Brunetti’s surprise, the woman, introducing herself, calls a completely different name - Maria Testa. It turns out that she left the monastic order after a series of deaths that happened in the hospital, which seem suspicious to her. The commissioner decides to check whether Maria's fears are well-founded, or whether she is deliberately exaggerating to justify her flight from the harsh monastic life.