The Man in the Iron Mask is indeed a continuation of The Three Musketeers by Alexandre Dumas. While it's often published separately, it is part of the d'Artagnan Romances sequence and follows the adventures of the Musketeers in later years. The story involves a mysterious prisoner whose identity is connected to the King of France. The Three Musketeers introduces d'Artagnan and his friends Athos, Porthos, and Aramis, and The Man in the Iron Mask continues their story in a later period.