Ireland, AD 671. The Great Fair of Bealtain is almost upon the fortress of Cashel, and a line of painted wagons carries entertainers to mark the occasion. But preparations take a deathly turn when one carriage is set alight, and two corpses are found, lying poisoned, within.

As Sister Fidelma and her companion, Eadulf, investigate, they are plunged into the menacing marshlands of Osraige - where the bloody origin of the Abbey of Cainnech is wreaking his revenge.

Fidelma and Eadulf must face untold mortal danger before they can untangle the evil that strikes at the very heart of the kingdom.