Man In The Iron Mask by Andrew Lang

Set in seventeenth-century France under an absolute and increasingly despotic monarch, the story follows the discovery that a mysterious prisoner, forced to wear an iron mask, is the king’s secret twin; the aging musketeer d'Artagnan and his former companions become embroiled in a desperate plot to replace the tyrant with his more virtuous brother, a plan that tests their loyalty, honor and friendship and ultimately collapses into tragedy and bitter reflection on power, identity and the costs of restoring justice.