Set in the 1620s, the novel opens with D'Artagnan, an honest young man from Gascony. He plans to begin a military career in Paris. D'Artagnan is from a noble family but not a very wealthy one. To Paris, he’s taking a letter of introduction written by his father for Monsieur de Treville, who is captain of the Musketeers. The letter is meant to ensure his spot in the elite squad that is the Musketeers, but on his way to Paris, d'Artagnan is beaten up and his letter is stolen from him. The people who stole it, Milady de Winter and Rochefort, do so thinking d'Artagnan is actually a spy.
Once he gets to Paris, d'Artagnan cannot join the Musketeers right away because he lost his letter, but he is still allowed an interview. When the interview is being conducted, d'Artagnan comes to realize the growing rivalry between the King backed by his Musketeers, and Cardinal Richelieu, a bright, young politician at the height of his career backed by his own Guard. D'Artagnan decides he will be on the side of the Musketeers and the King. His interview gives d'Artagnan a place in the Royal Academy, and it is also free of cost. Here he can learn to fence and ride, amongst other skills, and one day he can expect to join the Musketeers as one of them.
Treville is now writing d'Artagnan a new letter of introduction when d'Artagnan looks out the window to see the very man who stole his first one. Just a few short hours after his acceptance into the coveted institute, when d'Artagnan is trying to get to Rochefort to get his first letter back, he manages to offend three of the Musketeers at once. The first, Athos, is a middle-aged aristocrat who has a wounded shoulder. D'Artagnan collides with Athos’s shoulder, making it hurt even more. The other two musketeers he embarrasses. Porthos is a big man who doesn’t have wits going for him, and he also doesn’t have any money. Right now he is living off an elderly woman he has seduced. D'Artagnan brushes against him, which ends up uncovering a fake gold shoulder belt he has on. The last musketeer is Aramis. A brilliant man once on his way to becoming a Jesuit priest, Aramis is skilled in both the art of seduction and swordsmanship. D'Artagnan angers Aramis by stupidly calling people’s attention to a lady’s handkerchief. All three of the men want to duel d'Artagnan, so they all come together at an appointed time. Athos is up first, but when he begins to duel d'Artagnan, the Cardinal’s Guards appear and threaten to arrest them because of the new law against dueling that he has set. Out of a sudden impulse, d'Artagnan helps the three Musketeers fight off four of the Cardinal’s Guards. This experience binds the men together and from that point on begins a friendship that makes them inseparable for a long time to come. Consequently, d'Artagnan is awarded a rank in Monsieur des Essart's King's Guards very soon after he has arrived in Paris.
D'Artagnan has been staying in an inn where he ends up falling in love with the elderly innkeeper’s young and beautiful wife, Constance Bonacieux. Constance is a servant of Queen Anne and works in the palace. Sometimes she is sent off to do errands, but she has always been witnessing and hence knowing more than she should as a commoner.
What most people don’t know is that the Queen has been having an affair with the Duke of Buckingham. The Cardinal is looking for evidence of this affair because he hopes to embarrass the Queen, who embarrassed him when she denied his romantic interest in her. The innkeeper comes to ask for d'Artagnan’s help one day because his wife has been kidnapped by people he assumes to be the Cardinal’s men so that they can extract information from her about the Queen and her secrets. D'Artagnan saves Constance and this is when he falls in love with her exactly. Sometime after, he notices her walking with a strange man and so stops to talk to them. The man it turns out is the Duke the Queen is in love with, but because he is from England, he is automatically an enemy of the country of France. The Queen gives her lover a box of twelve diamond tags that night.
The Cardinal comes to know of this exchange from one of his many spies scattered throughout the palace and city. Once he finds out, he passes this knowledge along to the King, Louis XIII, who is now suspicious and asks the Queen to wear her diamond tags to a ball that is about to happen. Constance comes to d'Artagnan with the problem and asks for his help in getting the diamonds back for the Queen. To do so, d'Artagnan, the three Musketeers, and each of their servants all set off for England. In the end, only d'Artagnan and Planchet, his servant, make it through the difficult journey. When trying to get to England, d'Artagnan hurts the Count de Wardes badly. Finally, d'Artagnan is able to warn the Duke of Buckingham about what has happened back in France. This is when the Duke finds out two of the twelve tags have been stolen from him by Milady de Winter, and so he can’t return them. The Duke now commands the ports to be closed in England and instructs his personal jeweller to make exact copies of the diamond tags that can be used as replacements and attached to the rest. He gives d'Artagnan the twelve tags as well as horses that can complete the journey back to Paris in 12 hours. D'Artagnan comes back in time for the Queen to be able to wear all twelve tags to the ball. To thank him the Queen gives d'Artagnan a diamond ring.
Constance Bonacieux is kidnapped again by the Cardinal’s men. D'Artagnan comes across Milady de Winter and begins to fall in love with her. He pretends to be the man she is in love with, the Count de Wares and visits her in the dark of the night and makes love to her. Milady is so happy she gives d'Artagnan, who she thinks is the Count, a sapphire ring. D'Artagnan forges a second letter and pretends that the Count has sent Milady a letter rejecting her. Now furious, Milady hatches a plan to kill the Count and she recruits d'Artagnan to help her, this is when he reveals to her it was really him all along. Now that he is the subject of her fury instead, Milady wants d'Artagnan dead and attacks him. This is when the shoulder of her nightgown is torn and the mark of a convict is revealed. Milady is Athos’ wife whom he thought dead. D'Artagnan escapes.
Amidst the siege of La Rochelle, Athos and d'Artagnan sell the sapphire ring for equipment for the war. The ring actually belonged to Athos’ family and he had given it to his wife. Porthos gets his equipment for the war from the elderly mistress who has been keeping him. Finally, Aramis gets his equipment from a friend of his, Madame de Chevreuse. The Musketeers are out enjoying themselves while drinking at an inn when due to coincidence they happen upon the Cardinal making his way to Milady, who he is going to meet at the inn that they have just exited. The four of them follow the Cardinal and eavesdrop on the conversation between him and Milady when he tells her to kill the Duke. They steal the pardon he writes for her and go on to warn the Duke. He orders her to be locked up but she gets a guard to do her bidding and kill the Duke. Milady comes back to France and meets Constance hiding out on the Queen’s orders outside Paris in a convent. Here, she talks to Constance and befriends her. Then she kills Constance just before the arrival of the Musketeers by poisoning her, all for the sole reason that it will upset d'Artagnan. The Musketeers finally catch her after this and they call the executioner of Lille to make her answer for all her crimes throughout her life, from before she met Athos up to the deaths of the Duke and Constance. The executioner beheads Milady.
When they come back to La Rochelle, the four of them are caught and brought in front of the Cardinal by Rochefort, which is when d'Artagnan cleverly takes out the pardon that the Cardinal had written for Milady before. The Cardinal can’t do anything but finally acknowledge that he has been defeated, so he writes a letter for one of the men to be a Musketeer lieutenant. However, the three Musketeers present all have plans to go away and take time for themselves, and the one who ends up with the letter is d'Artagnan. He is one of the youngest of the rank, receiving the coveted commission at only twenty-one years of age.