In seventh-century Ireland, then the light of reason in a darkening world, Fidelma - a religious, advocate of the Brehon courts, and sister to the King of Muman - is an increasingly notable figure often called upon for her wisdom, legal knowledge, and devotion to the truth. While journeying with companion Eadulf to visit the new Archbishop of Canterbury, their ship is blown badly off course and the pair find themselves on the coast of modern Wales in the kingdom of Dyfed.

There, hosted by the king himself, Fidelma is presented with her knottiest problem yet - an entire monastic community from the nearby countryside has disappeared without a trace. While investigating this seeming impossibility, Fidelma and Eadulf are confronted with the apparently unrelated and shocking murder of a local girl - a death marked by more tragedy and consequence than anyone could have imagined.