The body of homeless child Benji Thatcher is discovered after his abduction, and his younger sister remains missing. When officials show little interest, Sebastian traces the children's movements and finds evidence that other vulnerable boys and girls have disappeared in similar circumstances.
The investigation leads from rookeries and brothels to aristocratic pleasure houses influenced by the writings of the Marquis de Sade. Sebastian realises that a protected predator is using wealth and status to obtain victims, and that rescuing the missing girl will require confronting men the law is reluctant to touch.