The story opens with Aramis (one of the musketeers who is now a priest) taking the last confession of a prisoner who is condemned to be executed soon. His confession comes as a thunderbolt to the former musketeer. This mysterious person is actually the identical twin brother of Louis XIV. He was ordered to be killed by his own parents, Louis XIII and Anne of Austria, to prevent any future challenge to the throne. However, he is ultimately thrown into prison and has lived there ever since. Now that Louis has ascended the throne, he is condemned to die. Aramis is not one to let such a crime go unpunished. He enlists the help of Porthos. He feels that Phillipe being the older twin and thus entitled to reign should be restored to the throne and along with this, Aramis himself can also become the Cardinal of France. Devious plots are meanwhile underway, launched by the evil Superintendent of Finance, Fouquet. Aramis brings Phillipe to the palace, kidnaps Louis and substitutes one twin for another. And the story races to its gripping climax.
In The Man in the Iron Mask, the final moments of some of musketeers is depicted very poignantly. Alexandre Dumas' story is based on some interesting historical facts. A mysterious prisoner called Eustache Daugher was held in several prisons across France and Italy, for 34 years under the custody of the same jailer who moved with him constantly. The French novelist Voltaire was the first to speculate about the possible identity of Daugher. However, nothing has been proven conclusively, though the story provided rich material for many conspiracy theories and novels.