The Good Soldier (S01E04)

When Aramis' old friend Marsac, a former Musketeer, unexpectedly returns to Paris the Musketeers are thrown into turmoil fully aware that they should report him for desertion. His appearance coincides with the royal visit of the King's beloved sister and her husband, the Duke of Savoy, whom the Musketeers must protect. Marsac believes he finally knows the truth behind the infamous massacre which killed his troop and wants Aramis' help to prove it. Despite their misgivings, will the Musketeers help and can they prevent Marsac exacting his own revenge? (BBC America)