Detective Cassie Maddox is still trying to deal with the events of In the Woods. She is out of the Murder Squad and has started a relationship with fellow detective Sam ONeill but is too badly shaken to commit to Sam or to her career.
Then Sam is allocated a new case, that of a young woman stabbed to death just outside Dublin. He calls Cassie to the murder scene and she finds the victim is strangely familiar. In fact, she is Cassies double. Not only that, but her ID says she is Lexie Madison the identity Cassie used, years ago, as an undercover detective.
With no leads, no suspects and no clues, Cassies old undercover boss spots the opportunity of a lifetime: to send Cassie undercover in the dead girls place. She could pick up information the police would never hear and tempt the killer to finish the job. So Cassie moves into Whitethorn House, poses as a post-grad student, and prepares to enter Lexies world.