Alexandre Dumas

Colbert has an intense hatred for M. Fouquet, the king's superintendent of finances, and has resolved to use any means necessary to bring about his fall. With the new rank of intendant bestowed on him by Louis, Colbert succeeds in having two of Fouquet's loyal friends tried and executed. He then brings to the king's attention that Fouquet is fortifying the island of Belle-Ile-en-Mer, and could possibly be planning to use it as a base for some military operation against the king. Louis calls D'Artagnan out of retirement and sends him to investigate the island, promising him a tremendous salary and his long-promised promotion to captain of the musketeers upon his return.
At Belle-Isle, D'Artagnan discovers that the engineer of the fortifications is, in fact, Porthos, now the Baron du Vallon, and that's not all. The blueprints for the island, although in Porthos's handwriting, show evidence of another script that has been erased, that of Aramis. D'Artagnan later discovers that Aramis has become the bishop of Vannes, which is, coincidentally, a parish belonging to M.
Fouquet. Suspecting that D'Artagnan has arrived on the king's behalf to investigate, Aramis tricks D'Artagnan.