In the late autumn of 666 A.D., Fidelma of Cashel - an advocate of the Brehon Courts, sister to the King of Cashel, and religieuse of the Celtic Church is at a crossroads. Needing to reflect upon her commitment to the religious life and her relationship to the Saxon monk Eadulf, she leaves Eadulf behind and joins a small band sailing from Ireland on a pilgrimage to the Shrine of St. James in modern-day Spain. Her first surprise on-board is the appearance of Cian, her first love, a man who had deserted her ten years ago, and who stirs up memories she'd rather forget.
As if this wasn't complication enough, on the first night out the ship is tossed by a turbulent sea and a pilgrim disappears, apparently washed overboard. But the appearance of a blood-stained robe raises the possibility of murder and death continues to dog the tiny band of pilgrims trapped within the close confines of the ship. Battling against the antagonism of her fellow pilgrims, Fidelma is determined to solve this most perplexing of puzzles before the ship reaches the shrine and the killer, if there is one, disappears forever.