The Musketeers must risk everything to save France from Rochefort's treacherous plan.

King Louis tasks the Musketeers with bringing his exiled brother, the Duke of Orléans, back to Paris in order to mend bridges. On the way, the Duke is robbed of dangerous letters which could destroy Orléans. The Duke informs the corrupt Governor of this, and the Governor's Red Guard must race the Musketeers in the hunt for the thief.

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.