Louis XIII had to be a Solomon. His queen produced twin male heirs - two sons of equal age with equal pretensions. Louis feared for France. One prince means peace and safety for the state, two are a recipe for civil war and anarchy. One son had to go. Louis could not kill him, but he could hide him in an iron mask. Such is the quality of paternal mercy.
This book was originally written as part of a sequel to The Three Musketeers. But in time, it developed a life of its own, and in recent years has always been published as a separate novel that stands on its own merits.