Two 14th century knights of the Crusade—Behmen von Bleibruk (Nicolas Cage) and his friend Felson (Ron Perlman)—both disillusioned with the slaughter of innocents, leave the Church's army. Passing through a town ravaged by plague, they are recognized by the priest Debelzaq (Stephen Campbell Moore) and brought before Cardinal D'Ambroise (Christopher Lee), who offers not to have them executed for desertion on the condition that they transport a young girl (Claire Foy), accused of being the witch who is causing the Black Death, to the Abbey of Severac, where the monks possess the last copy of the Clavis Salominis Regis (The Key of Solomon), an ancient book of rituals that will destroy the witch's powers and end the plague.

Season of the Witch is based on a spec script by screenwriter Bragi F. Schut. It's said to have drawn inspiration from the 1957 Swedish film The Seventh Seal (1957) (The Seventh Seal), which is also set during the Black Plague.

Known by various names such as "four thieves vinegar masks", "beak doctor costumes", or "plague doctor costumes", the bird-beaked masks were developed in medieval times to combat the plague, believed to be caused by miasma (bad air). The beaks were filled with vinegar-soaked sponges, dried flowers, essential oils, garlic, and aromatic herbs and spices. As shown in the movie, the entire plague doctor costume consisted of the beak mask, an ankle length overcoat, gloves, and wide-brimmed hats (to indicate that they were doctors).

Debelzaq keeps reading from the Key of Solomon, hoping to weaken the demon, but the dead monks begin to rise and attack them. For every monk that Behmen and Felson behead, another shows up to replace them, "like cockroaches," Felson says. One of the monks stabs Felson in the back, while the demon snaps Debelzaq's neck, so Kay takes up reading from the book. When the demon goes for Behman's throat, Felson staggers over to pull him off, but the demon encloses him in his wings and burns him to a crisp. Behmen pins the demon against the wall and tells Kay to keep reading. When Kay finishes the prayer, the demon explodes. Having been stabbed several time in the back by the demon's barbed wing, Behmen finally collapses. In another room, the girl coughs and awakens. Before he dies, Behmen orders Kay to keep the girl safe. In the final scene, Kay and the girl bury Debelzaq, Felson, and Behmen. "My name is Anna," the girl says over their graves, "I owe you my life. God keep you." Anna and Kay mount their horses and begin to ride away, the book safe in Kay's saddlebag. "Will you tell me about them?", Anna asks, "I want to know them." As they ride off together, Anna says in a voiceover, And so the plague passed and life returned to the land. There are many who say the plague was nothing but a pestilence that finally broke like the passing of a fever. They don't know the darkness that almost was, the sacrifices made, the heroes lost. I will tell their story. I was there. I know.

Not very. For example, the dates of the battles depicted at the beginning of the movie have been radically altered in order to coincide with the dates that the Black Plague ravaged Europe in the mid-14th century. For example, the real Siege of Tripoli lasted from 1102 to 1109, not in 1334 as stated in the film. Likewise, the real Battle of Artah was fought in 1105, not in 1339. The Battle of Smyrna was fought in 1344 but was not part of the Crusades, which ended in 1291. They were correct about accused witches being hanged rather than all being burned at the stake (as many people believe).