An Ancient Evil inaugurates an ingenious new series of historical mysteries by P. C. Doherty, each one using characters from Chaucer's Canterbury Tales. In Chaucer's poem, the pilgrims narrate tales to pass the time during their journey to the sacred shrine of Canterbury; Doherty's pilgrims entertain their companions with stories of mystery, terror, and murder. The Knight's Tale begins with the story of an ancient evil: the destruction of a sinister vampire cult that thrived in the wilderness of Oxfordshire during the reign of William the Conqueror. The tale then leaps two hundred years ahead to Oxford, where students are disappearing and citizens are murdered in a bizarre and brutal fashion. The sheriff of the city and the university authorities are baffled, but Lady Constance, Abbess of the Convent of St. Anne's, believes the murders are connected with the legends of the cult, and petitions the king for help. Two investigators, special commissioner Sir Godfrey Evesden and royal clerk Alexander McBain, arrive in Oxford and begin an investigation. The Archbishop of Canterbury sends unusual assistance in the person of the blind exorcist Dame Edith Mohun who, as a girl, had a horrifying experience with what may have been a remnant of that ancient cult. What this trio discovers and the repercussions for the city and the university form the scarily delightful plot of this novel.