In the summer of 2006, Rob Reilly, a smart-suited homicide detective, and his partner Cassie are dispatched to investigate a child's murder, and they find a community caught between old and new Ireland. On an altar lies the body of a local teenage girl, the precociously talented Katy Devlin. Her body is found in the middle of an archeological site, threatened by local developers aiming to build a shiny new motorway. The neighboring estate, Knocknaree, has never quite gotten its share of the “Celtic Tiger” and has been blighted by poverty and unemployment for generations. Moreover, this is the not the first time a child of Knocknaree has been lost — 21 years earlier, in a very different Ireland, three children went missing, and only one ever came back alive. Memory runs deep in this part of the world, and locals, press and the Dublin Police worry that the cases are linked.

Despite Rob's promise to Cassie that they won't investigate Operation Vestal because of Rob's intense relationship with Knocknaree — the long shadow cast by that August day when two children disappeared in the woods and were never seen again — he convinces her that they are the best people to solve this case. They make a dark bargain — to keep Rob's secret because he has insider knowledge of that estate and he believes the two cases could be linked. Now, she and Rob are locked into a secret and Operation Vestal is born. As the investigation gets underway, what is increasingly undeniable is that there is an atmosphere around Knocknaree and the Devlin family, and we start to get to know the potential suspects. Just as Rob and Cassie feel they are making progress; Cassie has her own visitation from the past.

All of Knocknaree mourns as Katy Devlin is laid to rest. The pressures on Rob and Cassie mount as there are still no arrests in the case.

"Frank outlines his larger plan for Cassie to go undercover as Lexie in order to find her double's murderer. Cassie turns him down flat. Meanwhile, the Katy investigation grinds to a halt, and Rob focuses his increasingly disturbed mind on tracking down Jonathon's two best friends from the past because he is convinced they hold the answers to both cases. As the pair are overwhelmed with memories of their traumatic childhoods, Rob and Cassie turn to one another for comfort and reassurance. But Rob cannot deal with any intimacy, and he is absolutely savage in his rejection of her. Reeling and feeling betrayed, Cassie agrees to Frank's plan to go undercover and play the role of the murdered young woman."

Cassie immerses herself in the art of being Lexie, the history of Whitethorn House and the backgrounds of the dead woman's housemates, who may or may not be murderers. Now alone, Rob makes costly mistakes and rapidly spirals. But in the depths of his despair, he remembers a fragment of his past that he is convinced is the key to everything. At Whitethorn House, the housemates — Daniel, Rafe, Justin and Abby — are told that Lexie is fragile but she is coming home to them. The four potential murderers stand on the steps of Whitethorn House, the grand house in Wicklow, watching the car approach. Cassie steps out, pale and thin, her stab wound dressed and healing, with a gun strapped to her leg.

Undercover as Lexie, Cassie performs all the tasks of the investigation into Lexie's murder. But she surprises herself when she is utterly emotionally seduced by the house along with Daniel, Rafe, Abby and Justin—Lexie's devoted friends. Just as she starts to question if Lexie's murderer is actually in the house, an attack on the housemates comes from outside. Meanwhile, Rob reaches out to Sandra, Jonathon's ex-girlfriend, the rape victim from his recovered memory. Sandra denies it was rape, and Rob is powerless to use his unique knowledge to pursue Jonathon as the prime suspect in both cases. When the suspect he arrested is cleared, Sam is upset that Cassie chooses to remain undercover, but Cassie is relieved. Right now, it is easier to be Lexie than it is to be Cassie.