In 1355, Margaret of Ashbury hears a mysterious voice advising her to do something unusual and possibly heretical; write a book about her life. She depends on renegade friar Brother Gregory to record her life story and state of Mystic Union, and he is forced to accept the state of grace of this "mere woman." Since she is unlettered herself, Margaret enlists Brother Gregory, a Carthusian friar, to take on the task, something he would not have done were he not afraid of starving. Still he cannot help but become engrossed in Margaret's fascinating life story, which includes midwifery, witchcraft, and the Black Death.