In the 13th century, three women accused of witchcraft are hanged by a priest. While one claims to be a witch out of persuasion from the church, one doesn't deny it and curses the priest. He kills them by hanging and implores the guards to pull them back up for a ritual that will make the so-called witches never come back to life, but the guards refuse, claiming them to be dead enough. Afraid that they would come back, the priest returns late at night alone and performs the ritual to prevent the bodies from coming back to life. While successfully completing the ritual for the first two of the bodies, the third one, who happens to be the one who claimed to be a herbalist, takes on a demonic appearance and kills the priest.Later, in the 14th century, Teutonic Knights Behmen Von Bleibruck (Nicolas Cage) and Felson (Ron Perlman) are engaged in a crusade, taking part in several different battles throughout the 1330s and eventually taking part in the Smyrniote crusades. After witnessing the massacre of civilians during the 1344 capture of Smyrna, the two knights choose to desert the Order and the crusade and return to Austria.While traveling through Styria, they encounter the grotesque sight of people infected with the Black Death and soon discover that the Holy Roman Empire has been swept by the plague. Behmen and Felson enter the medieval town of Marburg, the German name for Maribor (once part of Styria, now in Slovenia). The two try to conceal their identity as deserters but are revealed as knights by the crest on Behmen's sword. He and Felson are then arrested. They are taken to Cardinal D'Ambroise (Christopher Lee), who is infected with the plague.The Cardinal asks the knights to escort an alleged witch suspected of causing the plague to a remote monastery where an elite group of monks reside. These monks are capable of determining if the girl is truly a witch. If she is found guilty, the monks know a sacred ritual that can cancel her powers and stop the plague that is devastating Europe. The two knights agree under the condition that she will be given a fair trial and that the charges of desertion against them are dropped. The Cardinal agrees and they set out accompanied by a priest Debelzeq (Stephen Campbell Moore); Kay Von Wollenbarth (Robert Sheehan), a young altar boy who wants to become a knight like his deceased father; Eckhart (Ulrich Thomsen), a knight whose family were killed by the plague; and the well-traveled swindler Hagamar (Stephen Graham) who is serving as their guide to the monastery in return for a pardon. The witch, a young girl later identified as Anna (Claire Foy), shows hatred towards Debelzeq and forms a bond with Behmen.Shortly after setting off, the group camp for the night. Eckhart volunteers to watch the witch for the first night. After a while, Debelzeq comes to relieve Eckhart, who decides to remain with him. He tells Debelzeq about his daughter Mila, who resembled Anna. When Eckhart gets up to leave, Anna becomes hysterical at the prospect of being left alone with the priest. She attacks him and grabs his key to the cage. She escapes and flees toward a nearby village.The search for her leads the group to a mass grave, where Eckhart has visions of his dead daughter. Running after the visions, he impales himself on Kay's sword and dies. When they recapture her, the tearful Anna explains that she only ran away for fear of Debelzeq. However, the group becomes less trusting of Anna. The group manages to cross a rickety rope bridge, during which Anna saves Kay from falling to his death by grabbing him with one hand, showing an unnatural strength. The group enters the dark forest called Wormwood, where Hagamar attempts to kill Anna so the group can go home, only to be stopped by the others. Anna appears to summon wolves, who chase the group and kill Hagamar. An enraged Behmen tries to kill Anna, but is stopped by Debelzeq and Felson, who point out that the monastery is in sight.Arriving at the monastery, the men find all of the monks have been killed by the plague but locate the Key of Solomon, an ancient book filled with holy rituals used to defeat evil. The men confront Anna, with Debelzeq beginning to perform a ritual used on witches. However, as Anna begins precisely recounting Behmen's past actions during the Crusades, Debelzeq comes to the realization that she is not a witch and begins frantically performing an exorcism.However, the demon that is possessing Anna reveals itself and melts the metal of the cage. The demon effortlessly fights off the knights, but when Debelzeq throws a vial of holy water on it, it flies out of sight. As the men search for the demon, they come to the realization that it isn't trying to escape, but trying to destroy the book so that nothing can stop it.When they find a room where the monks were writing copies of the book, the demon appears, destroys the copies and possesses the dead monks' bodies to use as weapons. The three men fight the possessed monks while Debelzeq continues the exorcism ritual. During the fight, the demon kills Debelzeq, then proceeds to kill Felson. Kay picks up the book and continues the ritual, while Behmen continues fighting the demon. Behmen is mortally wounded during the fight, but Kay is able to finish the ritual and the demon is obliterated, freeing Anna. Behmen asks Kay to keep Anna safe and then dies of his wounds.Kay and Anna bury their fallen friends. Anna requests that Kay tell her about the men who saved her. They depart from the monastery with the book in hand.