Christmas day, and DCI Tom Reynolds receives an alarming call. A mass grave has been discovered on Oilean na Caillte, the island which housed the controversial psychiatric institution St. Christina's. The hospital has been closed for decades and onsite graves were tragically common. Reynolds thinks his adversarial boss is handing him a cold case to sideline him.
But then it transpires another body has been discovered amongst the dead - one of the doctors who went missing from the hospital in mysterious circumstances forty years ago. He appears to have been brutally murdered.
As events take a sudden turn, nothing can prepare Reynolds and his team for what they are about to discover once they arrive on the island . . .