Anno Domini 1327: The English Franciscan monk William of Baskerville travels to a rich Benedictine abbey in Italy on behalf of the Emperor. He is to mediate in a meeting between the Minorites, suspected of heresy, and the Pope's envoys. But at the same time a gruesome series of murders shakes the monastery and William sets out to find the culprit. Not everyone agrees with his investigations, for when the papal delegation arrives, William himself is accused of heresy and murder.