Medieval physician Kathryn Swinbrooke is brought back to the town of Canterbury, this time to investigate a recent plague of rats. Whether or not their visitation is the result of the town's moral or spiritual lack is unclear, but when miracles begin to surround the local beatified friar's corpse Kathryn suspects that something more threatening is afoot. She soon discovers that the friar was in fact murdered, and what began as a search for the town's ills becomes a desperate search for a killer.