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.

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...