It is the Year of Our Lord 1275, Santes Creus monastery, Spain. Brother Lucas, a venal but moderately trustworthy monk, is put in charge of an exorcism. His former fellow acolyte, the brooding and magnetic aristocrat Francisco 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. Gradually, Francisco comes out of his possession and begins to spin the tale of his Crusade: How he set out to free the soul of his dead brother lost at sea before he could reach the Holy Land; his fierce friendship with his cousin Andres, a mighty warrior with an honest heart; his intense love for Andres's feisty sister, Isabel; the high ideals of his battle-hardened commander Ramon; and the amoral cruelty of the perfidious Don Fernando, a noble who delights in executing Muslim women and children. The Crusaders win a great battle at Toron, taking it back from the Infidel, but at the gigantic fortress Krak des Chevaliers, the key to the Christian position in the Holy Land, the Saracens have laid siege. In the fierce battle to defend the fortress, Francisco will lose everything: including, unless the dodgy Brother Lucas succeeds, his immortal soul.