In Venice, the Three Musketeers Athos (Matthew Macfadyen), Porthos (Ray Stevenson) and Aramis (Luke Evans), with the help of Athos' lover, Milady de Winter (Milla Jovovich), steal airship blueprints made by Leonardo da Vinci. However, they are betrayed by Milady, who gives the blueprints to the Duke of Buckingham (Orlando Bloom). Upon returning to France, the Musketeers are forced to disband by Cardinal Richelieu (Christoph Waltz) for their failure.One year later, the young D'Artagnan (Logan Lerman) leaves Gascony for Paris in hopes of becoming a Musketeer, like his father once was, only to learn that they no longer exist. D'Artagnan ends up challenging Captain Rochefort (Mads Mikkelsen), the leader of Richelieu's guard, to a duel after being offended by him, but Rochefort merely shoots him while he's distracted. In an attempt to get revenge, D'Artagnan offends Athos, Porthos and Aramis for petty reasons, and schedules duels with each of them, at the same day and at the same place, but in different times, not knowing who they are.Before they can duel, however, D'Artagnan and the Musketeers are attacked by the guards for breaking the law by having a public duel. They fight the soldiers off, at which point D'Artagnan discovers their true identities, but end up being captured and brought before the young King Louis XIII (Freddie Fox) and his wife, Queen Anne (Juno Temple). Richelieu attempts to convince them to execute the four prisoners, but they are too impressed, and congratulate them instead, much to Richelieu's anger.Later, Richelieu meets with Milady, who is actually working for him. He orders her to plant false love letters among Queen Anne's possessions and steal Queen Anne's diamond necklace and hide it in the Tower of London with the objective of framing Queen Anne of having an affair with the Duke of Buckingham, who is in France on behalf of the King of England, and who has built a fully armed airship using the designs stolen from the Musketeers. The affair would force King Louis to execute Queen Anne and declare war on England. At this point, the people would demand a more experienced leader for the country: Richelieu himself. Before leaving, Milady demands that Richelieu gives her an authorization declaring that she was working on behalf of France's best interests.However, Queen Anne's lady-in-waiting Constance Bonacieux (Gabriella Wilde) discovers his plan and pleads with the Musketeers to stop Richelieu. They follow Milady and Buckingham to London, while Constance is kidnapped by Rochefort for helping the Musketeers to escape from him. Meanwhile, King Louis finds the false letters and is advised by Richelieu to set up a ball in which Queen Anne would be forced to wear the necklace. If she doesn't, then her affair is real, and there will be war.In London, Milady warns Buckingham of the Musketeers arrival, claiming that they want revenge for being outsmarted by Buckingham one year prior. Buckingham captures D'Artagnan and prepares to interrogate him when D'Artagnan reveals that he was acting as a decoy to allow the Musketeers to steal Buckingham's airship. They rescue D'Artagnan and capture Milady, who gives them the authorization in an attempt to have her life spared. Upon realizing she failed, she jumps out of the airship into the English Channel.The Musketeers recover the necklace and return to Paris, only to be attacked by Rochefort, piloting an airship secretly built by Richelieu, who was given copies of Da Vinci's blueprints by Milady. Rochefort feigns an attempt to exchange Constance for the necklace in order to capture D'Artagnan, but the Musketeers come to his rescue and the two ships crash in the Notre Dame Cathedral, where D'Artagnan fights and defeats Rochefort, rescuing Constance, who returns the necklace to Queen Anne.The Musketeers arrive at the ball and, for the sake of King Louis' and his people, lie by saying that Rochefort was trying to sabotage an airship that Richelieu built for them, for the purpose of identifying a traitor. To convince King Louis, Athos presents Milady's authorization, which King Louis accepts. Richelieu, satisfied, offers the Musketeers a place in his army, but they refuse, which infuriates Richelieu, who swears revenge.Meanwhile, at sea, Milady is rescued by Buckingham, who reveals that he intents to avenge her and destroy the Musketeers. It is revealed that Buckingham is advancing towards France with a massive fleet of airships and sea-faring ships.