It's a prequel to "Wicked", telling the story of Elphaba's father, Dr. D. D. D. W. (Double Dee Double You) Winkie, a brilliant and eccentric scientist, and his relationship with the future Wicked Witch of the West.
The book delves into Elphaba's early life, her father's obsession with science and his attempts to understand her unique abilities, and the political climate of Oz leading up to the events of "Wicked".