In January of 668 A.D., Fidelma of Cashel - sister to the king of Muman, an advocate of the Brehon law courts and a religieuse in the Celtic Church - is called to investigate the brutal murder of Abbess Faife and the mysterious disappearance of six young female religieuse accompanying  her on a short pilgrimage away from her abbey. When Sister Fidelma and her husband,  Brother Eadulf, arrive they find that there has been another death under mysterious circumstances, one of the senior scholars of the abbey has been bludgeoned to death. These two seemingly unrelated deaths - and the further mysteries of the trade ship lured to its doom on the rocky western shore of Ireland and the rumored figure of "The Master of Souls" wrecking havoc and raising rebellion - combine to create on the the most perplexing mysteries ever faced by the redoubtable Fidelma.