The Year of Our Lord, 1275, Santes Creus monastery, Spain. Brother Lucas, a venal and moderately trustworthy monk, is put in charge of an exorcism. His former fellow acoltye, the brooding and magnetic aristocrat Francisco de Montcada, has returned from the Crusades possessed by demons. If Brother Lucas can drive out the demons, his monastery will be enriched by the Montcada patriarch and he himself will be made a bishop.