Two nuns are brutally attacked within the walls of their convent. There seems to be no shred of motive. But during the autopsy Forensic Pathologist Maura Isles discovers something entirely unexpected. And when a second, heavily mutilated body is found and linked to the case, she and Detective Jane Rizzoli find themselves in the midst of a terrifying investigation that seems to implicate everyone. Because who can really say they're free from sin?