In the year 664, in the kingdom of Northumbria, the followers of the Church of Rome are fighting against the teachings of the Celtic Columban of Iona. To resolve the church dispute, a synod is convened in Witebia. When the abbess is found murdered in her cell and shortly thereafter two more servants of God die, church political motives are initially suspected. Sister Fidelma, an Irish nun of royal blood and also a lawyer in her homeland, investigates these rumors and makes a gruesome discovery.