Dumas constructs the plot around the notion that the Man in the Iron Mask is the twin brother of Louis XIV, Philippe, who had been concealed and imprisoned from birth by his father, Louis XIII, and his mother, Anne of Austria, "for the good of France". Only a very few people living at the start of the novel know of Philippe's existence; these include his mother, Anne, and her former confidante, the Duchesse de Chevreuse. Chevreuse has let the secret slip to Aramis when they had an affair. (Excerpt from Wikipedia)