In 1625 France, D'Artagnan leaves his family in Gascony and travels to Paris to join the Musketeers of the Guard. At a house in Meung-sur-Loire, an older man derides D'Artagnan's horse. Insulted, D'Artagnan demands a duel. The older man's companions instead beat D'Artagnan unconscious with a cooking pot and a tong that breaks his sword. His letter of introduction to Monsieur de Tréville, the commander of the Musketeers, a King's elite regiment, is stolen. D'Artagnan resolves to avenge himself upon the older man, who is actually the Comte de Rochefort, an agent of Cardinal Richelieu, who is passing the latter's orders to his spy, Milady de Winter.
In Paris, D'Artagnan visits Tréville at the Musketeers' headquarters. Without the letter, he faces a lukewarm reception from Tréville. Before their conversation concludes, D'Artagnan sees Rochefort passing in the street through Tréville's window and rushes out of the building to confront him. Pursuing Rochefort, he separately offends three musketeers, Athos, Porthos and Aramis, who each demand satisfaction; D'Artagnan must fight a duel with each of them that afternoon.
As D'Artagnan prepares himself for the first duel, he realizes that Athos's seconds are Porthos and Aramis, who are astonished that the Gascon intends to duel them all. As D'Artagnan and Athos begin, Richelieu's guards appear and attempt to arrest the musketeers for illegal dueling. Offered to leave by the Cardinal's guards, D'Artagnan decides to help the musketeers. Despite being outnumbered four to five, the four men win the battle. D'Artagnan seriously wounds Jussac, one of Richelieu's officers and a renowned fighter. King Louis XIII appoints D'Artagnan to Des Essart's company of the King's Guards, a less prestigious regiment, and gives him forty pistoles.
D'Artagnan hires a servant named Planchet and finds lodgings with Bonacieux, a merchant. His landlord later mentions the kidnapping of his wife, Constance Bonacieux, who works for Queen Anne of France. When she is released, D'Artagnan falls in love at first sight with her. Queen Anne secretly meets the Duke of Buckingham, England's first minister. At the meeting, she gives him a diamond necklace, the King's gift to her, as a keepsake.
Richelieu, who wants to diminish the influence of Queen Anne and her Spanish entourage on French internal affairs, plots to persuade the King that his wife is having an affair with Buckingham. On his advice, the King demands that the Queen wears the diamonds to an upcoming soirée. Constance tries to send her husband to London to fetch the diamonds, but he is instead manipulated by Richelieu and thus does not go, so D'Artagnan and his friends intercede. En route to England, Richelieu's henchmen attack them and only D'Artagnan and Planchet reach London. Before arriving, D'Artagnan is compelled to assault and nearly to kill Comte de Wardes, a friend of Richelieu, cousin of Rochefort and Milady's love interest. Although Milady stole two of the diamond studs, Buckingham provides replacements while delaying the thief's return to Paris. D'Artagnan thus returns a complete set of jewels to Queen Anne in time to save her honor.
D'Artagnan hopes to begin an affair with the grateful Constance. Invited to a date, he sees signs of a struggle and discovers that Rochefort and Bonacieux, acting under the orders of Richelieu, have kidnapped Constance. D'Artagnan traces his steps back to find his friends whom he abandoned wounded on his way to London. At their meeting, Athos, drunk, tells D'Artagnan a story about a count who fell in love with and married a young woman. Months later, the count discovered that his wife was branded with a fleur-de-lis on her shoulder, a punishment for felony. The count left her to die in a forest with her hands tied, abandoned his family castle and joined the King's guard under another name. D'Artagnan understands that Athos is telling his own story.
In Paris, D'Artagnan meets Milady and recognizes her as one of Richelieu's agents. He becomes infatuated with her, though her maid reveals that Milady is indifferent towards him. Entering her quarters in the dark, he pretends to be Comte de Wardes, whom she invited in a letter that D'Artagnan intercepted and makes love to her. However, D'Artagnan is not content with Milady's having sex with him thinking that he is de Wardes. He fakes a rude letter from de Wardes, offending Milady. She asks D'Artagnan to duel and kill the Comte. As a prepayment, he has sex with her again, without assuming a fake identity. In the heat of passion, D'Artagnan reveals that it is not the first time they are together. Milady is enraged and in the subsequent scuffle, D'Artagnan discoveres a fleur-de-lis branded on her shoulder. Milady attempts to kill D'Artagnan, who eludes her. He later tells Athos that his former wife is alive.
Cardinal Richelieu offers D'Artagnan a career in his guards' ranks. Dreading the prospect of losing his friends, D'Artagnan refuses despite understanding that his career prospects diminish as a result. With their regiments, D'Artagnan and the three musketeers are ordered to the Siege of La Rochelle. There, the four friends survive two assassination attempts by Milady's agents. The would-be assassins die in the process.
At an inn, Athos overhears Richelieu asking Milady to murder Buckingham, whose support is critical to the Protestant rebels at La Rochelle. Richelieu gives her his order absolving the bearer from any responsibility, but Athos takes the order from her. To get time to secretly consult with his friends, Athos bets that he, D'Artagnan, Porthos, and Aramis will hold the recaptured St. Gervais bastion against the rebels for an hour next morning. They resist for an hour and a half before retreating, killing a dozen Rochelaise in the process, which adds to their legend. They warn the Queen and Lord de Winter about Milady's plan to assassinate Buckingham. Milady is imprisoned on arrival in England, but seduces her guard, Felton,[a] and persuades him to allow her to escape and to kill Buckingham himself.
D'Artagnan is informed that the Queen has rescued Constance from prison. He gets a permission to take her from a convent where the Queen sent her to hide.
Upon her return to France, Milady hides, coincidentally, in the convent where Constance is hiding. The naïve Constance clings to Milady who pretends to be another victim of the Cardinal's intrigues. Seeking revenge on D'Artagnan, Milady poisons Constance before he arrives to rescue her. The musketeers catch Milady before she reaches Richelieu. Summoning a local executioner, they put Milady on trial, sentence her to death, and have her executed. The executioner reveals that it was he who branded Milady as a felon years before after she, a young nun at the time, seduced and then abandoned his brother, a local priest.
When the four friends return to the Siege of La Rochelle, Richelieu's Guards arrest D’Artagnan. D'Artagnan gives the Cardinal the secret order absolving the bearer of any responsibility which Athos had taken from Milady. Impressed with D'Artagnan's candor and secretly glad to be rid of Milady, Richelieu destroys the order and writes a new one, giving the bearer a promotion to lieutenant in Tréville's company, leaving the name blank. D'Artagnan offers the letter to his three friends in turn, but each refuses it; Athos because it is beneath him, Porthos because he is retiring to marry his wealthy mistress, and Aramis because he is joining the priesthood. D'Artagnan, though heartbroken and full of regrets, receives the promotion he had coveted.
