Athos, Porthos and d'Artagnan have been at war, while Aramis has been at the monastery caring for the children left orphaned. Missing munitions brings the three to the monastery where the bad guys have taken refuge. As they scheme to stop the stolen munitions from being sold to the Spanish and save the Monks and the children, Aramis is forced back into battle and has to decide where he truly belongs.

Refugees from the war are targeted by Feron for stealing grain in a plot to force the King to pay a higher price to feed the citizens of Paris. The Musketeers must work with one of them, Sylvie, to prove their innocence.

King Louis orders the Musketeers to bring his exiled brother, the Duke of Orléans, back to Paris. On the way, the Duke gets 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.

On her journey to Paris the queen of England, Henrietta Maria, also the sister of King Louis is robbed of the crown jewels, which has to be sold in order to finance England's armed forces. The Musketeers quickly discover the thief to be Émile Bonnaire, a flamboyant but pathetic criminal they encountered previously. Athos, Porthos and D'Artagnan pressure Bonnaire into leading them to the diamonds' buyers to recover them. Aramis on the other hand, 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.

A riot is taking place at the prison, the king has his hands full with his son's birthday.

Feron forges a letter supposedly from Treville sending the four musketeers on an errand, intending that they will be murdered on the way but before they set out Athos survives an attempt on his life by Grimaud, who also springs Gaston from jail, whilst the king asks Aramis to accompany him on a pilgrimage to his father's grave. On their own Porthos and D'artagnan must weather an attack until their comrades rescue them after which the normally obtuse king, having realized the Dauphin's parentage, nonetheless joins with Aramis and the other musketeers to defeat his would-be killers, including Feron. However Grimaud escapes with Gaston, whom he intends to crown king as he believes Louis to be terminally ill.

With the king taking to his bed as dying the queen asks Aramis to help broker a peace treaty with her brother, the Spanish king and he sets out to meet the ambassador but is captured by Grimaud. The court then receives an ultimatum - the return of Aramis in exchange for captured Spanish soldiers, a ploy engineered by Grimaud, Gaston and the Duke of Lorraine to discredit the queen as an appeaser. Facing popular acrimony the other musketeers have the prisoners released and set out for the rendezvous, along with D'Artagnan's cousin Espoir, whom he has saved from the gallows and acquits himself well in the gun-battle. Aramis is freed but Grimaud escapes to use the queen's sponsorship of a literacy scheme for the poor against her and Athos's new love Sylvie whilst an old enemy of the musketeers returns to the court.

Treville takes controversial steps to protect the Dauphin as France's enemies draw closer. Will his secretive tactics drive the 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.