The Musketeers Season 1 Episode 4

When Aramis's old friend, Marsac (a former Musketeer), unexpectedly returns to Paris believing that he finally knows the truth behind the massacre in which his troop was killed. He comes to the Musketeers seeking Aramis's help to prove it. The Musketeers are thrown into turmoil, knowing that they're duty bound to report him for desertion. Marsac arrives at the same time as the king's sister and her husband, the Duke of Savoy, of whom the Musketeers must protect. Despite their misgivings, the Musketeers decide to help, but can they prevent Marsac from exacting his own revenge?