The book is centered, once again around the Three Musketeers. The narrative of the full volume takes place between 1660 and 1667 during the ascension of the young king Louis XIV to the throne of France. The titular character, the man in the iron mask, is supposed to be the twin brother of Louis XIV himself who was imprisoned with his identity concealed by an iron mask by his mother and father. Few people are aware of the twin, Philippe's existence until his mother's friend the Duchesse de Cheveruse lets the secret slip out to her former lover, Aramis, the musketeer. Aramis creates a plan to replace Louis with Philippe in order to create a more susceptible king that he can use as a puppet. But when his plan begins to backfire more than just Aramis will pay for his avarice.