In Venice, the musketeers Athos, Porthos, and Aramis, with the help of Milady de Winter, steal airship blueprints made by Leonardo da Vinci. However, they are betrayed by Milady, who incapacitates them and sells the blueprints to the Duke of Buckingham.
A year later, d’Artagnan leaves his village in Gascony for Paris in hopes of becoming a musketeer as his father was, only to learn that they were disbanded. At a rural bar, he challenges Captain Rochefort, leader of Cardinal Richelieu’s guard, to a duel after being offended by him, but Rochefort merely shoots him while he’s distracted. Once in Paris, d’Artagnan separately encounters Athos, Porthos and Aramis and, accidentally offending all three, schedules duels with each. Athos brings Porthos and Aramis to the duel as his seconds and d’Artagnan realises who they are. Richelieu’s guards arrive to arrest them, but, inspired by d’Artagnan, the musketeers fight together and win. All four are summoned before the young King Louis XIII and Richelieu urges him to execute them, but Queen Anne is impressed by their bravery and the king condecorates them instead.
Richelieu instructs Milady, now his accomplice, to plant false love letters among Queen Anne’s possessions, steal her diamond necklace, and take it to the Tower of London in order to frame her as having an affair with Buckingham, which would force King Louis to execute her and declare war on England. At this point, the people would demand a more experienced leader: Richelieu himself. In order to secure her own position, Milady demands that Richelieu declare in a written authorization that she is working on behalf of France.
The false letters are found and given to King Louis, who is advised by Richelieu to set up a ball at which Queen Anne would be forced to wear the necklace. If she doesn’t, then her affair is real, and there will be war. Queen Anne’s lady-in-waiting Constance Bonacieux discovers Richelieu’s plan and pleads with the musketeers to stop him. They follow Milady and Buckingham to London, while Constance is captured by Rochefort for helping the musketeers to escape from him.
In London, Milady tells Buckingham the musketeers have arrived to take revenge on him and teaches him all their tendencies in battle. D’Artagnan is captured, but turns out to be a decoy, allowing his associates to steal Buckingham’s airship and rescue him. Milady’s getaway coachman reveals himself as the musketeers’ manservant Planchet and delivers her to his masters, who retrieve the necklace from her. Athos prepares to execute Milady for her treachery, but she leaps off the airship, apparently dying on her own terms.