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.
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.
The Musketeers battle to restore order in Paris after Grimaud and Feron stage an ambitious Châtelet breakout, setting free a lunatic with murderous designs on the Louvre.
It's the day of reckoning, as Grimaud vows to kill the Musketeers in a series of savage co-ordinated attacks and puts Feron to the ultimate test in his bid for power. The furious battle for survival sparks a series of revelations that threaten to destroy Paris. Before the day is out, a hero falls, there’s a traitor on the rise and the Musketeers will be changed forever.
When death strikes the Louvre, Treville takes controversial steps to protect the Dauphin from the unholy alliance of Grimaud, Gaston and the Duke of Lorraine. His secretive tactics isolate the Queen and threaten to drive even his loyal Musketeers apart.
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.
