In Ireland of A.D. 666, Sister Fidelma is sent by her brother, Colgu of Cashel, the king of Muman, to the remote valley of Gleann Geis, whose inhabitants still adhere to the ancient Druidic ways. Her mission is to negotiate with the chieftain Laisre for permission to build a Christian church and school in his territory. Fidelma's task won't be an easy one, though, as Laisre's clan is known for its hostility to the new religion and fierce adherence to the old.
Approaching the valley, Fidelma and her companion, Brother Eadulf, come upon a particularly grisly scene—the slain bodies of thirty-three young men, placed in a sunwise circle and bearing the marks of the ancient threefold death of pagan times. As an emissary of her brother the king, as well as her position as a dalaigh—an advocate of the Brehon courts—it is Fidelma's responsibility to uncover the truth behind the gruesome murders. Within the forbidden valley, Fidelma embarks upon an inquiry that not only places her in the gravest personal danger but upon which rests the continuing peace of her brother's kingdom.