D'Artagnan, a poor but noble young man from Gascony, leaves his home to make his fortune in Paris; he is carrying a letter of introduction to his father's friend, Monsieur de Treville, captain of the King's Musketeers. On the way to Paris, d'Artagnan's impulsive nature gets him into trouble; he is beaten and the letter of introduction is taken from him. In Paris, he nevertheless is granted an interview with Monsieur de Treville, and is promised acceptance in the Royal Academy free of charge, where he can learn fencing, riding, and good manners; later, with experience, d'Artagnan, can expect to become a musketeer.
While Treville, is writing a new letter of introduction, d'Artagnan glances out the window and, by accident, sees the person who robbed him. He runs after him, and while pursuing him, he offends three musketeers: first, he collides with Athos, reinjuring Athos's wounded shoulder; then he jostles Porthos and reveals a partly counterfeit golden shoulder belt that he is wearing; and finally, he offends Aramis by ungallantly and unintentionally bringing attention to a lady's handkerchief. He is challenged to a duel by each of the musketeers. After he meets the musketeers and begins dueling with Athos, they are all threatened with arrest by the dreaded cardinal's guards because of a law against dueling. D'Artagnan joins forces with the musketeers and helps drive the cardinal's men away. Thus, almost immediately after his arrival in Paris, D'Artagnan becomes an intimate friend of the three musketeers.
One day, d'Artagnan's elderly landlord, Bonacieux, comes to ask him for help; the landlord's young wife, Constance, has been kidnapped — probably by the cardinal's men because she is the queen's linen maid and knows many of the queen's secrets, secrets which the cardinal desperately wants revealed so that he can discredit the queen, who earlier rejected his romantic advances. D'Artagnan is able to rescue Madame Bonacieux from her abductors and, while doing so, falls in love with her. Later, when he inadvertently sees her cross a bridge with a strange man, he stops them and discovers that the man is an English nobleman, the duke of Buckingham, the queen's secret lover; being an Englishman, the man is also an enemy of France. That night, the queen gives the duke an elegant gift of twelve diamond tags in a rosewood box.
When the cardinal, through his extended and vast network of spies (one of whom is among the queen's ladies-in-waiting) discovers that the queen has given Buckingham the diamond tags, he asks the king to give a fabulous ball and demand that the queen wear the king's gift to her: the twelve diamond tags.
The queen is terrified when she learns about the ball and hears her husband order her to wear the diamond tags. She knows very well that they are in London, in the possession of the duke of Buckingham. Meanwhile, the cardinal sends one of his spies — the elegant and beautiful Milady — to London; he instructs her to dance with the duke, snip off at least two of the diamond tags, and return them to the cardinal so that he can use them in a blackmail scheme.
Ready to help the queen regain the diamond tags, whatever the cost, Constance Bonacieux pleads with d'Artagnan to undertake the dangerous trip to London in order to retrieve the diamond tags from the duke before the ball and thereby save the queen's reputation. D'Artagnan readily accepts Constance's request, and accompanied by the three musketeers, he begins the hazardous trip to London. On the way, they are continually ambushed by the cardinal's spies, and one by one, the musketeers are foiled from accompanying d'Artagnan to London.
When d'Artagnan reaches London, he reports the situation to Buckingham, who discovers in horror that two of the tags are missing. Immediately, he calls in his personal jeweler and instructs him to work furiously in order to make exact copies. He gives the copies to d'Artagnan, along with the remaining ten tags, and a superb, prearranged series of horses that will take d'Artagnan from London to Paris in twelve hours. Thus, the queen is able to appear in what seems to be all twelve of the diamond tags — to the utter astonishment of the cardinal. For d'Artagnan's heroic efforts, the queen secretly presents him with a large, magnificent diamond ring.
After agreeing to a rendezvous with Constance (which never takes place because she is again abducted by the cardinal's men), d'Artagnan is told that it is dangerous to remain in Paris: the cardinal knows everything that happens in Paris; it will not be long, before he learns about d'Artagnan's role in the diamond tag escapade. D'Artagnan therefore decides that this would be a good time to discover what happened to his musketeer friends.
He returns to each of the places where he left them, and finding them all safe, they return to Paris — only to discover that they must buy equipment for the king's next military maneuver: the siege of La Rochelle. Each of the musketeers must find some way of getting money — something they are always short of.
While pondering how to get some cash, d'Artagnan sees Milady by accident and is overwhelmed by her beauty; he follows her and tries to protect her from a bothersome man who turns out to be her brother-in-law. The brother-in-law challenges d'Artagnan to a duel and they fight. D'Artagnan overpowers him, but spares his life. In appreciation for his life, the brother-in-law — Lord de Winter — introduces d'Artagnan to Milady, Lady de Winter. Meanwhile, Milady's maid sees d'Artagnan and falls in love with him, and later she tells him that Milady is madly in love with Count de Wardes, the man whom d'Artagnan wounded just before sailing to London. She also gives d'Artagnan a love note which Milady has written to de Wardes. D'Artagnan is so furious that he forges de Wardes' signature on a return letter to Milady, arranging a dark, nighttime rendezvous with Milady. While she thinks that she's making love to de Wardes, d'Artagnan will be making passionate love to her.
The plan works, and afterward Milady is so satisfied that she gives d'Artagnan an elegant sapphire ring surrounded with diamonds, promising to have "that stupid d'Artagnan" killed for having wounded de Wardes, the man she thinks she's been making love to in the darkness.
Later, d'Artagnan is furious, and, in order to get revenge against her, he answers another love note of hers to de Wardes, signing de Wardes' name under a flippant reminder that Milady has to "wait her turn." Milady is so incensed that she asks d'Artagnan to kill de Wardes, and as prepayment she goes to bed with him. d'Artagnan is so enamored by Milady's loveliness that he impulsively reveals that this is not the first time that he has made love to her: earlier, when she thought that she was making love in the dark to de Wardes — she wasn't. D'Artagnan was in bed with her.
Milady rears up and tries to kill d'Artagnan and as they scuffle, her nightgown is torn and d'Artagnan sees the mark of a convict branded on one of her shoulders. The discovery of this secret is so terrible that Milady vows that d'Artagnan will die. By a stroke of good fortune, however, and some help from Kitty, d'Artagnan escapes.
Relating the adventure to Athos later, the two men discover that Milady is Athos's wife, a woman whom he thought he hanged after he discovered that she was a branded criminal. Athos and d'Artagnan decide to sell Milady's "tainted" ring — which originally belonged to Athos's family — and now they are both able to buy their equipment for the siege of La Rochelle. Meantime, Porthos has obtained his equipment from his aging, miserly mistress, and Aramis has obtained his equipment from his beloved friend, Madame de Chevreuse.
Before d'Artagnan and the musketeers leave in their separate regiments for the siege, the king becomes ill, and d'Artagnan's group moves out first, leaving the musketeers behind for the time being to await the king. d'Artagnan is lonesome for his friends and, one day, he wanders off alone — not a wise decision, because he is fired at by two of Milady's hired assassions. Later, during a dangerous mission that d'Artagnan is leading, the same two assassins again try to kill him. When this attempt fails, Milady decides to have some poisoned wine delivered to d'Artagnan -compliments of "the three musketeers." D'Artagnan does not realize that the wine is poisoned, and he is so busy talking that he fails to drink the wine immediately. Instead, another soldier drinks the wine — and falls dead.
Meanwhile, the three musketeers are enjoying their leisure time, drinking and joking, and, by chance, they meet the cardinal, who is going to a meeting with Milady, who is staying at the inn which the musketeers just left. The musketeers accompany the cardinal and listen through a broken stovepipe to the conversation.
Milady, they learn, is going to London to make sure that the duke of Buckingham is killed; in return, the cardinal will take revenge against d'Artagnan. The musketeers immediately decide on a plan to warn d'Artagnan and Buckingham. Thus, when Milady arrives in England, she is taken prisoner by her brother-in-law, de Winter. However, she cleverly corrupts her jailer, convinces him (a religious puritan fanatic) that Buckingham deserves to be put to death, and he obeys her.
She then escapes to France, where she is determined to complete her revenge against d'Artagnan. She goes to the convent where the queen has placed Constance Bonacieux, d'Artagnan's beloved, for protection, and there Milady wins the young girl's confidence. Precisely when d'Artagnan and the musketeers arrive to rescue Constance, Milady poisons her and escapes.
D'Artagnan and the musketeers track her down, accuse her of her many crimes — and execute her. When the entire story is revealed later to the cardinal, he is horrified at the extent of Milady's evil web of death, and he is extremely impressed with d'Artagnan's laudable actions. Consequently, he writes out a commission for d'Artagnan to become a lieutenant in the King's Musketeers. After offering the commission to Athos, Porthos, and Aramis and being refused by all three, d'Artagnan accepts the prestigious commission at the early age of twenty-one.