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 find a community caught between old and new Ireland.
On an altar lies the body of a local teenage girl, 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 neighbouring estate, Knocknaree, has never quite got 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, 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's an atmosphere around Knocknaree and the Devlin family.
Just as Rob and Cassie feel they are making progress Cassie has her own visitation from the past.
Katy is finally laid to rest, and all of Knocknaree have come out for the funeral. The public and media pressure to find Katy's killer is intense, and Superintendent O'Kelly is concerned that all lines of inquiry have led to naught. Rob, however, is increasingly certain that his memories are leading him to the truth. As the partners dig into the past and pursue leads in the present, we learn about the dark ancient history of the woods, that Knocknaree means ‘hill of the king' and if the archaeologists can't finish their work, the truth about the place will be lost forever. As one lead goes nowhere but another starts to look promising, Cassie gets a call from Sam about another dead body.
Frank comes up with a plan for Cassie to go undercover as her double to discover who killed her, but she refuses to be involved. However, a savage rejection from Rob that leaves her feeling betrayed leads her to have a change of heart. As the investigation into Katy drags to a halt, Rob becomes fixated on tracking down two of Jonathan's old friends, convinced they are the key to solving both mysteries.
Rob, unmoored by Cassie's absence, is increasingly obsessed with Jonathan Devlin, convinced he and his childhood friend Cathal Mills may be dangerous paedophiles. He is desperate to find the one witness to the events in the woods in 1985 that he hasn't yet spoken to - itinerate ex-friend of Jonathan and Cathal's, Shane Waters. But Shane, a homeless drug addict, hasn't been seen since Katy's body was found.
Meanwhile, Jonathan has sent his daughters Rosalind and Jessica away from Dublin. Rob tracks Rosalind down to a dingy caravan park. Soon the extent of Rosalind's disturbed behaviour emerges, and Rob feels he can save the girl. Meanwhile, Cassie starts to back away from Operation Vestal, spending time with Frank as he schools her in the art of being Lexie.
Rob realizes time is running out on the investigation and decides the only way to fully remember what happened that day is to spend the night alone in the woods, totally immersed in that strange world...
Undercover as Lexie, Cassie finds herself emotionally seduced by the house and Lexie's devoted friends. She and Frank discover the locals in the village hate Whitethorn House and its owners because of what they represent - colonialism and cruelty, with the scars and skeletons of the famine cottages. Furthermore, the inhabitants of Whitethorn House are refusing to sell, blocking plans for a new motorway that would bring jobs to the area.
Meanwhile, Rob is obsessed and distressed by his memories of the day of the disappearances. The only thing he can do is to reach out to Sandra, the rape victim who still lives on the estate. But Sandra denies everything, throwing him out of her house.