The Vicomte de Bragelonne: d'Artagnan Romance Series, Book 3

It is May 1660 and the fate of nations is at stake. Mazarin plots, Louis XIV is in love, and Raoul de Bragelonne, son of Athos, is intent on serving France and winning the heart of Louise de la Valliere. D'Artagnan, meanwhile, is perplexed by a mysterious stranger, and soon he learns that his old comrades already have great projects in hand. Athos seeks the restoration of Charles II, while Aramis, with Porthos in tow, has a secret plan involving a masked prisoner and the fortification of the island of Belle-Ile. D'Artagnan finds a thread leading him to the French court, the banks of the Tyne, the beaches of Holland, and the dunes of Brittany.