As the leading churchmen and women gather at the Synod of Whitby in 664AD to debate the rival merits of the Celtic and Roman Churches, tempers begin to fray. Conspirators plot an assassination, while mysterious, violent death stalks the shadowy cloisters of the Abbey of St Hilda. When the Abbess Etain, a leading speaker for the Celtic Church, is found murdered suspicion inevitably rests on the Roman faction.
Attending the Synod is Fidelma, of the community of St Brigid of Kildare. As an advocate of the Brehon Court, she is called on to investigate the murder with Brother Eadulf, of the Roman faction. However, the two are so unlike that their partnership is described as that of a wolf and a fox - but which is which?
More gruesome deaths follow and the friction among the clerics could end in civil war. Can the solution to the mysteries avert such a conflict?