The brightest and best musketeers of the King's Regiment - Athos, Porthos and Aramis - are dispatched by Captain Treville to find a missing musketeer carrying important letters on behalf of King Louis. Meanwhile, d'Artagnan travels from his family farm in Gascony to Paris to petition the king, but his peaceful plans are turned upside down.

Vadim is determined to make life as unbearable as possible for the authorities that have imprisoned him, after he manages to organize a full-scale riot.

Assigned to escort privateer Emile Bonnaire from Le Havre to Paris the musketeers have to thwart an escape attempt by his wife Maria and an ambush by his aggrieved ex-business partner Meunier and his men, during which Porthos is wounded, necessitating a sojourn at a deserted château. Athos explains it belongs to his family and was where he married Milady. Bonnaire is revealed to be a slave owner and, on arrival in Paris, bribes Richelieu into forming a partnership operating tobacco plantations in the New World, using slaves as a work force. However there is another faction out to capture Bonnaire and the musketeers are braking no rules in handing him over to them.

The Duke of Savoy and his wife, the king's sister, arrive in Paris at the same time as Marsac, a former Musketeer, now a deserter, who seeks Aramis's help after an abortive attempt on the duke's life. Marsac's troops were massacred as they slept whilst on a mission to depose the Duke. Marsac's informant tells him they were betrayed to the Duke by Captain Treville, the leader of the Musketeers. Aramis discovers that all records of the night of the massacre have disappeared, whilst the duke suspects that Richelieu was behind it and has located a Spanish spy who can prove it. The Musketeers must prevent the Duke from starting a war with France, as well as proving their captain's innocence.

After a night of heavy drinking to celebrate his birthday, Porthos wakes in the street next to a corpse, with no recall of events. He is arrested and sentenced to hang but rescued by boyhood friend Charon and taken to the Court of Miracles, a sanctuary for law-breakers. The other Musketeers identify the dead man as Jean De Mauvoisin but find that he was killed some distance from where his body was found. It becomes apparent that Jean's father, in league with Charon, was planning to blow up the Court of Miracles for financial gain and Jean was silenced when he found out. With dozens of barrels of gunpowder installed at the court, the Musketeers must prevent a massacre as well as clearing Porthos's name.

King Louis's treacherous mother, the exiled Marie de' Medici, returns to seek protection from an assassin. Aramis and d'Artagnan sent to collect a young woman, Agnès, and her baby son are stunned when armed men abduct the baby and the Musketeers seek to find the baby and uncover who they were sent to escort.

The musketeers must save an enlightened woman who has been accused of witchcraft.

LaBarge, regional superintendent in Gascony, is arrested for killing two musketeers. Our friends are tasked to bring him safely to Paris. The Red Guards, who are tired of the Musketeers' treatment of LaBarge, provoke a confrontation between the two regiments. King Louis, who is tired of this rivalry, is organizing a competition to determine once and for all which of the two regiments is strongest. D'Artagnan takes on the challenge - but needs to work on himself that his temper should not go to his head - to show that he mastered the art of battle.

Frustrated by his wife's infertility, Louis makes a foolish statement, suggesting it were best if she were dead. As a result, Richelieu sends Irish gunman Gallagher and his gang to kill her while she is travelling with the Musketeers. Sending their friends back for reinforcements, Aramis and Athos take her to a convent for sanctuary, where Aramis is surprised to see his former love, Helene is one of the nuns, though she and the Mother Superior prove themselves very capable in the subsequent siege. However, it is with the queen that Aramis grows intimate, to Athos's horror. The siege is lifted, the queen saved and the king contrite whilst Richelieu manages to frame somebody else for the assassination attempt.

Milady seeks her revenge against the Musketeers. Will the Musketeers outwit her or will she destroy them by manipulating their weakest link; D'Artagnan?