In medieval England, a vain, wealthy young woman named Margery goes mad after the birth of her first child and is tormented by devils for half a year, eight weeks, and four days. Then one night, she is restored to sanity by a vision of Jesus Christ in purple robes. In gratitude, she devotes her life to God, becomes the acolyte of a local friar, and sets about trying to become a saint. Battling her intense appetites for earthly pleasures and her complete unsuitability for religious vocation, she becomes increasingly fanatical, alienating her friends and family and endangering her own life.