Dominican friar Etienne de Bourbon visits a 13th-century French village in search of heretics for the Inquisition. Despite the opposition of the local priest and the indifference of the villagers, he finds a seemingly perfect suspect: a young woman who lives in a forest outside the village and cures people with herbs and folk remedies. In the process, he discovers the cult of the greyhound "Saint" Guinefort, and confronts his own troubled past.