A young woman is brutally murdered in a New York bookshop; Detective Callum Doyle receives a call from her killer, telling him he's willing to give him clues as to the identity of his next victim - but only on condition that Doyle doesn't tell anyone. And so as the killings continue, Doyle must interpret the 'helper's' riddles in order to establish a link between his victims, and prevent more from being killed - while all the time trying to protect his career, and his family . . .