The Sinner involves Detective Jane Rizzoli and a character new to Gerritsen's novels, medical examiner Dr. Maura Isles. When a young novice nun about to take vows is found murdered in the abbey's summer chapel, Isles and Rizzoli are immediately called to the scene. The elderly nuns are of little help to Isles and Rizzoli but when another body is found, mutilated beyond recognition (and testing reveals the body to be that of a fortyish Indian Hansen's Disease victim), it is soon discovered that there is more to these killings than meets the eye.
Also, Rizzoli finds that she is pregnant, complicating matters both personally and professionally. The novel ends with the wedding of FBI agent Gabriel Dean, introduced in The Apprentice, and the 5-months-pregnant Rizzoli.