The sequel to Twenty Years After is titled Ten Years Later and consists of 288 chapters, which is divided into three parts. Part three is titled: The Man in The Iron Mask.
Philippe, the twin brother of Louis the King of France has been imprisoned since birth by his parents in order to prevent a dispute over the crown by twin brothers. The secret is revealed by the Duchesse de Chevreuse to Aramis however during an illicit affair.
Aramis, a bishop, plots to replace Louis with Philippe as a puppet ruler for himself and Fouquet, with the overall intention become the next Pope.