In the enchanting city of Venice, the legendary Three Musketeers—Athos, Porthos, and Aramis (played by Luke Evans)—join forces with Athos' lover, Milady de Winter (portrayed by Milla Jovovich), to steal crucial airship blueprints crafted by the brilliant Leonardo da Vinci. However, their scheme takes a treacherous turn as Milady betrays them, handing the coveted designs over to the scheming Duke of Buckingham (depicted by Orlando Bloom). Returning to France, the Musketeers face the wrath of Cardinal Richelieu (Christoph Waltz), who disbands them due to their failure.
A year passes, and young D'Artagnan (played by [Logan Lerman]) embarks on a journey from Gascony to Paris, fueled by his ambition to join the Musketeers, just as his father once did. To his dismay, he discovers that the Musketeers have been dissolved. In a moment of bravado, D'Artagnan challenges Captain Rochefort (Mads Mikkelsen), leader of Richelieu's guards, to a duel after being insulted. Unfortunately, Rochefort takes advantage of the situation, shooting D'Artagnan when he is distracted.
Seeking revenge, D'Artagnan unknowingly offends Athos, Porthos, and Aramis, scheduling duels with each of them on the same day and location. However, as they prepare for combat, they are ambushed by the guards for violating public duel laws. During the ensuing chaos, D'Artagnan uncovers their true identities just before they are captured and presented to the young King Louis XIII (Freddie Fox) and his queen, Queen Anne (Juno Temple). Richelieu, aiming to execute the Musketeers for their defiance, is thwarted as the king and queen are impressed by their bravery, leading to compliments instead of condemnation.
In a clandestine meeting, Richelieu aligns himself with Milady, who is in fact his accomplice. He instructs her to fabricate love letters among Queen Anne’s belongings and to steal her diamond necklace, sequestering it in the Tower of London, with the intention of framing Queen Anne for infidelity with the Duke of Buckingham. This dangerous ploy could incite war between France and England, allowing Richelieu to seize power as the people's desired leader. Before her departure, Milady insists on receiving a formal note from Richelieu acknowledging her service to France.
Meanwhile, Queen Anne's devoted lady-in-waiting, Constance Bonacieux (Gabriella Wilde), uncovers Richelieu's machinations and urgently implores the Musketeers to intervene. They embark on a perilous mission to London, where they must confront the Duke and rescue Constance, who is captured by Rochefort for aiding their escape. Meanwhile, Richelieu discovers the falsified love letters and advises King Louis to host a ball, intending to force Queen Anne to wear the stolen necklace, thus entraping her in a scandalous affair.
In London, Milady warns Buckingham of the Musketeers' approach, claiming they seek vengeance for past grievances. When D'Artagnan is captured, he cunningly reveals that his capture was a ruse to allow his comrades to retrieve Buckingham's airship. They successfully rescue him and take Milady captive, who, in desperation, provides them with the authorization from Richelieu. Realizing she'd been outmaneuvered, she leaps from the airship into the depths of the English Channel.
With the necklace recovered, the Musketeers return to Paris, where they encounter Rochefort, who piloted his own airship built using Richelieu's designs. Rochefort pretends to negotiate Constance's release in exchange for the necklace but is met with resistance. A fierce battle ensues, culminating in a dramatic clash at Notre Dame Cathedral, where D'Artagnan bravely defeats Rochefort and rescues Constance, who promptly returns the necklace to Queen Anne.
Arriving at the ball, the Musketeers protect Louis and his realm by fabricating a story about Rochefort's attempt to sabotage Richelieu’s airship—an act aimed at exposing a traitor. Presenting Milady’s authorization as proof, they sway King Louis to their narrative, much to the satisfaction of Richelieu, who then offers the Musketeers positions in his army. However, they resolutely decline his offer, inciting Richelieu's wrath and pledging revenge.
Meanwhile, on the open sea, Milady is rescued by Buckingham, who reveals his intentions to retaliate against the Musketeers. As they plot their course, Buckingham's formidable fleet of airships sets sail towards France, setting the stage for a breathtaking confrontation.
