On the Spanish front, Athos, Porthos, and D'Artagnan find themselves outgunned when a supply train bearing gunpowder doesn't come through. They go to investigate and discover that a new player, Lucien Grimaud, is responsible. Meanwhile, Minister Treville and Constance deal with the new leader of the Red Guards, Louis' stepbrother the Marquis de Feron.

Louis has the Musketeers bring his traitorous brother Gaston to Paris, despite the latter's exile, and extends the hand of friendship to the relative who tried to overthrow him. Meanwhile, the veterans of Paris grow restless when Louis refuses to pay their pensions. When Gaston loses a purse containing incriminating letters, Feron sees an opportunity to benefit himself.