Ireland, AD 671. Fidelma undertakes a secret mission whose purpose she has sworn not to reveal, even to Eadulf. At the abbey of Finnbarr, the abbot she intends to question is murdered before he can speak, and a young woman accused of the crime disappears.
Rumours then claim that the kings of Cashel are plotting against the High King and his wife, placing Fidelma's family and her own life in danger. Bound by her oath, she must pursue the fugitive and the abbot's hidden knowledge while Eadulf struggles to help without understanding what she has been sent to uncover.