Whicher,retired from the police after his failure to solve the Road Hill House case,encounters Susan Spencer in a rough tavern,seeking her niece Mary,who came to London to find Stephen Gann,father of her baby. Soon afterwards Mary is found slain and the child missing and Whicher offers to help Susan find the killer. The baby is located at a home for unwed mothers,run by Reverend Marlow,who believes Mary went after Stephen,who rejected her. Whicher learns that Stephen's father was hung for murdering Susan's father though Mary knew nothing about it. Stephen and an opportunistic thief are the main murder suspects but Stephen convinces Whicher that he was the intended victim as he was about to hear important news from his grandfather Joshua,now in an insane asylum. Whicher inveigles his way into the asylum to speak to Joshua and,in solving the case,uncovers dark secrets in the Gann and Spencer families as well as betrayal by a corrupt policeman...