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.