Athos, Porthos and D'Artagnan are fighting on the battlefront while Aramis has trouble living in contemplation as a monk. The Musketeers' search for missing gunpowder lead them to follow bandits to Aramis' monastery, where circumstance force them to collaborate to save the monastery's children and brothers. Aramis' finally accepts his true calling as a Musketeer and rejoins his brothers-in-arms in Paris.

The corrupt Governor of Paris conspires with the Duke de Beaufort, primary provider of food for Paris, to stage a robbery of the city's granary and pin it on the refugees, only to later sell the stolen grain back to the King at an inflated price. D'Artagnan is taken captive along with the rioting refugees, and the other Musketeers (along with D'Artagnan's wife Constance, now in charge of the Musketeer garrison) must rescue them from the hands of the Red Guard before they are executed.

Following Treville's funeral the musketeers survive an attempt by Grimaud to kill them when their garrison is torched, leaving many casualties, and gunpowder and weapons stolen. To lure the musketeers into a trap Grimaud takes hostages including Sylvie but the four comrades stage a rescue, and the queen and Dauphin are restored to the palace with Milady as the royal spy master, her first task being to take care of Gaston. However, the mortally wounded Grimaud escapes and the musketeers must stop him from blowing up the city with the stolen gunpowder.