Based on an ancient Anglo-Scots ballad called "The Famous Flower of Serving Men" (a magical, tragic tale about a young woman who disguises herself as a man after the murder of her husband), Through a Brazen Mirror tells the tale of the witch Margaret, her daughter Elinor, King Lionel, and the extraordinary William Flowers, who saves the king from doom and Margaret's malevolent magic. With much richness of detail and folklore, Sherman uses the tale to explore issues of attraction, loyalty, and gender identity. For William is Elinor, who as William bonds with the young king and takes charge of a fate that was to be her undoing.

The book, though it takes place in a fictional kingdom, is no cleaned-up fairy tale, and presents a historical picture of life circa 1400. In an age when women are not taught to read or write, both Margaret, as an evil schemer, and Elinor, who must take on a man's name and countenance, fight for their self-determination, each in her own way.