In the late summer of 1472, medieval physician and apothecary Kathryn Swinbrooke is summoned to investigate yet another puzzling situation in Canterbury. She is appointed by the Archbishop as Advocatus Diaboli-the Devil's Advocate-to argue against the beatification of Roger Atworth, a friar in the Order of the Sack and the confessor of Dame Cecily of York, King Edward's mother. Atworth has died under mysterious circumstances, and there are rumors afloat of miraculous happenings surrounding his body. At the same time, an infestation of rats has begun to afflict Canterbury. Never in the history of the city has such an invasion occurred, and the topic is one everyone's lips.Meanwhile, when Kathryn begins asking questions at the friary about Atworth's death, she discovers that the logical explanation is murder, not a miracle. His involvement with Cecily may have brought about his demise, and Kathryn suspects a link between his death and that of an English spy outside the friary. With the murderer still on the loose, what began as a search for the town's ills becomes instead Kathryn's pursuit of a killer.