The Three Musketeers is the story of d’Artagnan, a young man from Gascony who dreams of becoming a musketeer (a soldier who carries a rifle). At the beginning of the novel, he sets off from his hometown and makes his way to Paris where he plans to introduce himself to M. de Tréville, the king’s righthand man and the leader of the musketeers. On his way to Paris, he picks a fight with a Rochefort, one of the cardinal’s best men, although d’Artagnan doesn’t know who he is at the time. Upon arriving in Paris, he makes his way to M. de Tréville, who gets him started on the path to becoming a musketeer.
Shortly after his meeting with M. de Tréville, d’Artagnan challenges three different musketeers—Athos, Porthos, and Aramis—to a duel, not realizing any of them are friends. When d’Artagnan shows up to his duel with Athos, he is surprised to see that Athos brought Porthos and Aramis as his seconds. However, before their duel can begin, the cardinal’s men interrupt them and pick a fight with the musketeers. Realizing that his issues with the musketeers are silly, d’Artagnan decides to ally himself with them against the cardinal’s men. D’Artagnan and the musketeers easily win the battle against the cardinal’s men and from that moment on, the four men become great friends.
Shortly after his battle with the cardinal’s men, d’Artagnan becomes embroiled in a plot to save the queen from one of the cardinal’s schemes. Simultaneously, he falls in love with a woman named Madame Bonacieux who works for the queen. She asks d’Artagnan to deliver a letter to the Duke of Buckingham in England. Although d’Artagnan doesn’t know it, the queen gave the Duke of Buckingham some diamond tags (jewelry) as a token of her affection, but now, the queen needs them back—otherwise, the cardinal plans to humiliate her in front of the king by revealing her affair with the duke. Wanting to win Madame Bonacieux’s favor, d’Artagnan heads to London with Athos, Porthos, Aramis, and their servants by his side. On the way to London, Athos, Porthos, and Aramis stay behind at various points in the journey to deal with the cardinal’s traps. D’Artagnan makes it to London and gives the duke the letter. The duke returns the tags to d’Artagnan, who swiftly takes them back to Paris and gives them to the queen. Although this is a great success for d’Artagnan, it puts him on the cardinal’s radar. Because of this, M. de Tréville tells d’Artagnan that he must watch his back.
Not long after d’Artagnan returns from Paris, the cardinal kidnaps Madame Bonacieux, who now loves d’Artagnan in return. M. de Tréville promises to help d’Artagnan find her, but in the meantime, d’Artagnan must find out what happened to the musketeers. As quickly as he can, d’Artagnan sets off to find each of his friends. He does so without too much trouble, although ach of his friends is in a terrible emotional state that d’Artagnan must snap them out of. Athos, in particular, is in a bad mood and drinks a lot. One night while drunk, Athos tells d’Artagnan a story about his “friend” (who is clearly Athos himself). This friend apparently married a beautiful woman and then killed her after he found out she was branded with a fleur-de-lis. This story horrifies d’Artagnan, though he doesn’t know what to make of it.
Eventually, the musketeers make their way back to Paris. For the next few weeks, they prepare for the upcoming war against the English. Additionally, d’Artagnan becomes acquainted with Milady de Winter, a woman who he knows is friendly with Rochefort. At this point, d’Artagnan thinks Rochefort had something to do with Madame Bonacieux’s kidnapping and wants to find out more details. Although he initially plans to use Milady to get to Rochefort, d’Artagnan quickly finds himself falling in love with Milady. Eventually, d’Artagnan goes to bed with Milady after promising to fight a duel on her behalf. While in bed with Milady, d’Artagnan admits he’s been dishonest with her, and she gets angry with him. As he tries to calm her down, d’Artagnan rips Milady’s nightgown, revealing a fleur-de-lis on her shoulder. This revelation causes Milady to go berserk; she tries to stab d’Artagnan, who barely escapes with his life. He tells Athos about the encounter and though neither man says it out loud, both know that Milady is Athos’s wife.
Not long after this incident, d’Artagnan and the musketeers ride off to war. Because d’Artagnan is not a musketeer himself, he doesn’t fight alongside his friends. While at war, d’Artagnan feels isolated and afraid. He knows Milady might try to take revenge on him and indeed she does. She sends several assassins his way as well as some poison wine. Luckily, d’Artagnan foils her plans and is eventually reunited with his friends.
Not long after d’Artagnan and his friends are reunited, the musketeers overhear a conversation between the cardinal and Milady. The cardinal tells Milady that he will allow her to kill d’Artagnan with impunity if she assassinates the Duke of Buckingham. The musketeers tell d’Artagnan about Milady’s plan and the four of them decide to write one letter to the queen and one letter to Lord de Winter, Milady’s brother-in-law, warning them about Milady’s plan. Lord de Winter is d’Artagnan’s friend as well as a close confidant of the Duke of Buckingham. D’Artagnan knows Lord de Winter doesn’t like Milady and suspects that she wants him dead so that she can steal his money.
Milady sails to London, where she is promptly abducted by one of Lord de Winter’s men, John Felton, and taken to a castle. There, Lord de Winter explains that he plans to send Milady to a penal colony as soon as he can get a letter from the duke granting him permission. Until then, Milady must wait in captivity. During that time, John Felton, a highly religious man, watches over her. Slowly, Milady seduces Felton and wins him over to her side. She tells him an elaborate (and false) story about how the Duke of Buckingham raped and unfairly branded her, which makes Felton, who already despises the duke because of his religious beliefs, hate him even more. Just before Milady can be sent away, Felton breaks her out of her cell and takes her to London on a boat. While in London, Felton goes by himself to find the duke and kills him. He is captured in the process.
Milady makes her way back to France and eventually ends up in a convent. There, she meets Madame Bonacieux, who managed to escape the cardinal and has been hiding in the convent for some time. When Milady learns that Madame Bonacieux is d’Artagnan’s mistress and that d’Artagnan is coming to rescue her, she poisons her as an act of revenge. D’Artagnan finds Madame Bonacieux just in time to watch her die. Realizing what happened, d’Artagnan, who is now a musketeer himself, tracks down Milady with his friends’ help. They also bring along a local executioner who Milady wronged in the past. Together, the group holds a trial for Milady and declares her guilty. The executioner then beheads her.
Everyone heads back to Paris. The war is over for the time being and the cardinal makes d’Artagnan a lieutenant of the musketeers for his service. Although the cardinal does not like that d’Artagnan foiled some of his plans, he still got what he wanted, and he has great respect for d’Artagnan. He also introduces d’Artagnan to Rochefort and forces the two men to promise to get along. D’Artagnan and Rochefort comply, although clearly a rivalry still exists between the two of them. In the years after, d’Artagnan serves as a lieutenant for the musketeers. Gradually, all of his friends retire to marry and explore their passions.