A headless female corpse is found in the drinking well of a remote abbey in south-west Ireland. One hand clasps a crucifix; tied to the other arm is a pagan death symbol...

A merchant ship is encountered under full sail on the high seas off the Irish coast. But the crew and cargo have vanished - as if by sorcery... Whose is the body in the well? Where is the crew of the vessel? Are these bizarre events connected? And if so, who is responsible?

The year is AD 666, and Sister Fidelma of Kildare, advocate of the Brehon law courts, must follow a trail of clues and investigate a host of enigmatic suspects to unravel the puzzle.