Ireland, AD 666. King Colgú sends Fidelma to Gleann Geis to negotiate permission for a Christian church and school in a valley whose people remain attached to older religious customs. Before reaching the settlement, Fidelma and Eadulf find thirty-three young men arranged in a ritual circle and marked by the threefold death associated with pagan sacrifice.
The massacre makes a diplomatic mission into a murder inquiry in territory suspicious of both Cashel and the new faith. Fidelma must decide whether the ritual appearance is genuine or deliberately staged, while competing factions use the deaths to threaten the fragile relationship between the valley and her brother's kingdom.