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 O`Neill 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 Cassie`s 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, Cassie`s old undercover boss spots the opportunity of a lifetime: to send Cassie undercover in the dead girl`s 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 Lexie`s world.