Sister Fidelma has been sent by her brother, king of Cashel, to Laisre, chieftain of Gleann Geis - the 'forbidden valley' - to negotiate permission to build a Christian church and school in his territory, replacing the pagan Druidic sanctuaries.

Laisre is known to be hostile to the new religion, and Fidelma knows her mission will be no easy task. Entering Gleann Geis with the Saxon Brother Eadulf, she comes across the naked, slain bodies of thirty-three young men, positioned in a sunwise circle. Each body bears the marks of stabbing and garrotting; every skull has been smashed.

Who is reponsible for such evil, if not the heathen Laisre?