When a local Abbess is murdered, Sister Fidelma and her companion Brother Eadulf are called to investigate.

An Anglo-Saxon deputation arrives in Cashel to debate the new religious rules from Rome with an Irish delegation. The Abbot of Imleach leads the Irish delegation who are hostile to the new rules, and among the Anglo-Saxon delegation is Brother Eadulf's own younger brother, Egric. There is also an observer appointed from Rome - the Venerable Favorinus.

The debate is bad tempered and acrimonious, and local Abbess Dar Oma has to step in as a mediator between the two sides. But that evening her body is discovered, she has been bludgeoned to death.

The animosity between the two sides is heightened as the Chief Brehon Aillin accuses young Egric of murder. The Venerable Favorinus counter claims that the murderer is Brother Madagan, who is steward to Abbot of Imleach. Suspicions and tempers run high.

With the war of words threatening to spill over into bloodshed, Fidelma is compelled to resolve the mystery as she is sure there is something more sinister behind the murder than religious differences.