Martin Horst, a Catholic soldier, is caught in the turmoil of the Thirty Years' War and suffers a fatal wound in the combat to take the Protestant town of Magdeburg. Dying and alone among the dead, he lingers in a peasant's field as his comrades storm the besieged city's broken walls. Horst seeks a meager stable's shelter and solace in his last moments. He meets a stranger, offering to save his life in exchange for rendering his services as a scribe. In the stable, an ass is slaughtered, its hide skinned, and Horst undertakes to pen the devil's bible on its foul surface. The writings unlock a nefarious calendar predicting the future and offering advantage and power to its possessor-Catholic or Protestant. Horst enlists the aid of a young monk, Friar Klanseck, questing to recover the hide and redeem his soul, as others learn of the vile skin's existence and contest them.