In September of 666 A.D., an aged monk and a set of relics disappear during the night from the Abbey of Imleach. The missing monk is a matter of great concern for the abbey. But the relics are a disaster of a much higher order. They are the priceless holy relics of St. Ailbe, the man who, in the fifth century, brought Christianity to the Irish kingdom of Muman, converted and baptized it's king, and founded the abbey. The relics themselves are the political symbol of the entire kingdom and their disappearance threatens to disrupt its continued peace and stability.

On a visit to the Abbey of Imleach, Sister Fidelma, sister to Colgu, the current King of Muman, and an advocate of the Brehon Courts, is asked to investigate. With the help of her friend and traveling companion, the Saxon monk Brother Eadulf, Fidelma begins to slowly unravel the mystery behind the twin disappearances. But there are dark forces at work, which will stop at nothing even murder to achieve their aims.