The main character, d'Artagnan, born into an impoverished noble family of Gascony, [1] leaves home for Paris to fulfill his greatest dream: becoming a Musketeer of the Guard. Fortunately his father knows Monsieur De Treville, Captain of the Company of Musketeers (and fellow Gascon) and has written a letter of introduction. On the road to Paris, the young Gascon soon gets in a quarrel with a mysterious gentleman and is set upon by the servants of the nearby inn. When d'Artagnan regains consciousness he realizes that the gentleman has stolen his letter of introduction. The innkeeper manages to get his hands on much of d'Artagnan's limited money as he recuperates.
In Paris d'Artagnan goes straight to M. De Treville's hôtel, but lacking his father's letter is received somewhat coolly. In a series of incidents at the hôtel, d'Artagnan is challenged to duels by three musketeers: Athos, Porthos, and Aramis. The four men meet and d'Artagnan begins to fight Athos (the first challenger). They are interrupted by Cardinal Richelieu's guards who threaten to arrest them because duels are forbidden by royal decree. The three musketeers and d'Artagnan unite to defeat the cardinal's guards. In this manner the young Gascon earns the respect and friendship of Athos, Porthos and Aramis and soon becomes a soldier in a regiment of the Royal Guard.
After obtaining lodging and hiring a servant (Planchet), he meets his aging landlord's pretty young wife, Constance Bonacieux, with whom he falls instantly in love. Constance and d'Artagnan help the Queen Consort of France, Anne of Austria, and the Duke of Buckingham have a rendezvous and the Queen presents her lover with a wooden box containing a set of diamond jewels originally given to her by her husband Louis XIII. Cardinal Richelieu, informed by his spies of the gift, persuades the King to invite the Queen to a ball where she would be expected to wear the diamonds; in hopes of uncovering her love affair.
Constance attempts to get her husband to go to London and retrieve the diamonds, but he has been recruited as an agent by the cardinal and refuses. D'Artagnan and his friends are convinced to take on the mission instead. After a series of adventures, they retrieve the jewels and return them to Queen Anne, just in time to save her façade of honour. Athos, Porthos and Aramis are all badly wounded by the cardinal's agents in this endeavor.
The cardinal's revenge comes swiftly: the next evening, Constance is kidnapped. D'Artagnan brings his friends back to Paris and tries to find her, but fails. Meanwhile, he befriends the Count de Winter, an English nobleman who introduces him to his sister-in-law, Milady de Winter. D'Artagnan quickly develops a crush on the pretty noblewoman, but soon learns that she has no love for him, being an agent for the cardinal. He manages to sleep with her and learns that Milady has a fleur-de-lis burned into her shoulder, marking her as a felon. She had apparently been married to both Athos and the Lord de Winter at different times in her wicked life and was livid that the young musketeer knew her secret. D'Artagnan is able to escape her home but is relieved when all the King's guards are ordered to La Rochelle where a siege of the Protestant-held town is taking place.
Milady makes several attempts to kill d'Artagnan in and around La Rochelle, but fails. At the same time, d'Artagnan finds out that the Queen has managed to save Constance from the prison where the cardinal and Milady had thrown her and that his beloved is now hidden somewhere safe. One of the would-be assassins drops a valuable tip: the name of an inn where Milady was to pay him for his crime.
The Musketeers stake out the inn and overhear a conversation between the cardinal and Milady, during which the cardinal asks her to assassinate the Duke of Buckingham (a supporter of the Protestant Rochellais rebels). The churchman then writes out a blanket pardon to Milady, effectively giving her permission to kill d'Artagnan. Athos quickly confronts his former wife and forces her to relinquish the cardinal's pardon. Because of the war between France and England, any attempt by the musketeers to warn the Duke of Buckingham about Milady would be considered treason, but they are able to send Planchet with a letter to Milady's brother-in-law (Lord de Winter) who suspects Milady killed his brother.
Milady is imprisoned on arrival in England, but soon seduces her hard-hearted Puritan jailer Felton and convinces him not only to help her escape, but also to assassinate the Duke of Buckingham. While the naive Felton knifes the prime minister, Milady sails to France. Milady sends a message to the cardinal and hides in the same North French monastery where Constance had been sent by the Queen. The trusting Constance bares her soul to Milady and the evil woman realizes that her enemy d'Artagnan is expected to arrive at the monastery at any moment. She escapes just before his arrival, but not before taking her revenge: she poisons Constance who dies minutes later in the arms of her beloved d'Artagnan.
They arrange to track down the whereabouts of Milady to exact punishment, joined by the Lord de Winter. The noblemen find her and try the countess on numerous charges: the poisoning of Madame Bonacieux; the assassination attempts on d'Artagnan; accomplice to the assassination of the Duke of Buckingham; the corruption of the Lord de Winter's servant, Felton; and the assassination of her late husband, Count de Winter. The most damning charge comes when Athos states that Milady, his wife, is a marked criminal with a brand on her shoulder. When the countess demands that Athos present the one who branded her, a man in the red cloak steps forward. She immediately recognizes him as the executioner of Lille and he recounts Milady's early misdeeds that led to the branding.
After Milady is beheaded the musketeers return to La Rochelle. On their way they encounter the Count of Rochefort, the cardinal's close advisor and d'Artagnan's old nemesis, who was traveling to Milady to pay her. Rochefort also has an order to arrest d'Artagnan. He decides to postpone his trip to Milady in order to take d'Artagnan directly to the cardinal. When the young Gascon is presented before him the entire story about Milady's assassination attempts, her poisoning of Madame Bonacieux, etc. is told. The cardinal states that if Milady is indeed guilty, the courts will deal harshly with her. d'Artagnan frankly admits that he and his friends have already dealt with this evil woman. He then presents Richelieu with the blanket pardon written in the cardinal's own hand. The cardinal, impressed by d'Artagnan's resourcefulness and having already gotten what he wanted from Milady, offers the young man a lieutenant's commission with the musketeers—with the name left blank. The cardinal then presents Rochefort and asks both men to be on good terms.
As the book's ends D'Artagnan offers the officer's commission to each of his friends, but is told that he should insert his own name. Athos intends to retire to his estates, Porthos has decided to marry the widow of a rich lawyer and Aramis will soon fulfill his dream of entering the priesthood.