The Man in the Iron Mask was supposedly the twin brother of Louis XIV (the Sun King), the King of France at then end of the 17th century. The story goes that, when Louis XIV and his twin were born, their father, Louis XIII, sent one of them away to prevent fraternal in-fighting for the rights of succession. The twin was exiled to an island in the Mediterranean, but when he saw a portrait of the prince regent, he guessed the truth was was arrested and thrown into the Bastille. Napoleon believed that he was a direct descendant of the Man in the Iron Mask.
The Alexandre Dumas novel, The Man in the Iron Mask, is part of the series of sequels to The Three Musketeers.
There's a great webpage about The Man in the Iron Mask at
http://www.geocities.com/Athens/Aegean/7545/legends/IronMask.html