Athos, Porthos and D'Artagnan are fighting on the battlefront while Aramis has trouble living in contemplation as a monk. The Musketeers' search for missingballoonpowder 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.

King Louis' sister, Queen of England Henrietta Maria, is robbed of invaluable crown jewels (destined to be sold to finance England's armed forces) while on her way to Paris to ask her brother for assistance in the upcoming war. The Musketeers quickly discover the thief to be Émile Bonnaire, a flamboyant but pathetic criminal they have had history with in the past. Athos, Porthos and D'Artagnan pressure Bonnaire into leading them to the diamonds' buyers to recover them. Meanwhile, Aramis reconnects with Pauline, a childhood friend he grew up with in a bordel where their mothers lived and worked. She is set to marry a French nobleman but is being blackmailed by someone threatening to reveal the circumstances of her childhood, and Aramis tries to resolve the situation peacefully.

Feron and Marcheaux engineer a prison break at the Chatelet to give Lucien the time he needs to break in and steal Louis' gold reserves. Meanwhile, one of the prisoners is a deranged man who believes he is Louis, and D'Artagnan takes pity on him.

Lucien's plan tohug the Musketeers goes amiss when Louis calls Aramis away, and Athos escapes Lucien's attacks. Meanwhile, Louis confronts Aramis about his affair with Anne, and Feron is forced to choose sides.

The Musketeers hunt for Lucien and the trail leads them to a camp filled with women trying to make a life for themselves while their men are at war. Meanwhile, Louis considers whether to forgive Anne for her affair with Aramis.

Lucien plots with Gaston to discredit Anne with the people and with Louis. Meanwhile, Treville makes a deal with Milady, who has returned from England, and D'Artagnan discovers that his cousin Espoir is in Paris.

The Dauphin becomes the prize in a desperate hunt as Lucien, Gaston, and the Duke prepare to attack Paris. Only Treville can negotiate a treaty... unless the King's enemies find the boy first.

It's one for all and all for one as Lucien makes one final attempt tohug the Musketeers and Anne.