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 of the carriages is set alight, and two corpses are found, lying poisoned, within. As Sister Fidelma and her companion, Eadulf, investigate, they are quickly plunged into the menacing marshlands of Osraige - where the bloody origin of the Abbey of Cainnech is wreaking his revenge.