For years the boglands of Northern Europe have given up bodies of the long-deceased. Bodies that are thousands of years old, uncannily preserved. Bodies with strange injuries that suggest ritual torture and human sacrifice.

When a corpse is found in a bog in Galway, Cormac Reilly assumes the find is historical. But closer examination reveals a more recent story. The dead man is Thaddeus Grey, a local secondary school principal who disappeared two years prior.

There's nothing in Grey's past that would explain why he was murdered, or why his body was mutilated in a ritual manner. At first, progress on the case is frustratingly slow and Cormac struggles to keep his mind on the job. His ex-girlfriend, Emma Sweeney, is in trouble, and she's reached out to him for help – Emma's new husband has gone missing in Paris, and the French police are refusing to open an investigation into his disappearance.

Cormac is sure that he has found Grey's killer, and is within hours of an arrest, when another mutilated body is discovered on the other side of the country. Two days later, a third body is found. Press attention is intense. Is there a serial killer at work in Ireland? Has Cormac been on the wrong trail? And if so, can he find the murderer before they strike again?