D'Artagnan meets Lord de Winter and three friends for their duel, with Porthos, Aramis and Athos as his seconds. The Englishmen force the Musketeers to reveal their true names--the Musketeers do so secretly, and Athos notes that he must now kill his opponent: he wants no one to know his true identity. The fight begins, and Athos kills his man. Aramis and Porthos defeat their opponents, and d'Artagnan disarms Lord de Winter, but spares him, declaring that he loves his sister. Lord de Winter is infinitely grateful, and promises to introduce d'Artagnan to Lady de Winter.
Athos is confused by d'Artagnan's behavior--he talks about being in love with Madame Bonacieux, and now he's obsessed with Lady de Winter. D'Artagnan tries to rationalize his interest by saying that he believes her to be a Cardinalist agent, but this excuse is only half true; something definitely draws him to this odd Englishwoman.
The next day, Lord de Winter introduces d'Artagnan to Lady de Winter. She is charming, but d'Artagnan is shocked by the faces she makes when she thinks no one is watching; her expression changes from that of a pleasant, beautiful hostess to a murderous animal, only to switch back the moment anyone addresses her. Confused, but more intrigued than ever, d'Artagnan starts calling on Milady every day. As d'Artagnan falls further in love with Milady, her maid, Kitty, begins to fall in love with d'Artagnan.
One day, Kitty takes d'Artagnan aside and confesses that she loves him. She also tells him that Milady does not love him, she loves the Comte de Wardes. D'Artagnan persuades Kitty to let him overhear Milady confessing her hatred of d'Artagnan, because if he had killed Lord de Winter, she would have inherited all the Lord's money. She says the only reason she hasn't "dealt with him" is that the Cardinal wants her to treat him carefully. She also mentions her involvement in the kidnapping of Madame Bonacieux.
All suspicions are now confirmed. Milady is a Cardinalist agent and a sinister villain. D'Artagnan is determined to have his revenge; he uses Kitty to intercept correspondences between Milady and the Comte. He sends her a false letter under the Comte's name, making an assignation to meet at her house. Kitty helps d'Artagnan because she loves him; for his part, d'Artagnan pretends to return her feelings, even having sex with her to make his act convincing.
D'Artagnan's plan is to compromise Milady, and then reveal his true identity to her, thus humiliating her and forcing her to tell him where Madame Bonacieux is. But when the hour of the tryst comes, d'Artagnan disguises himself as the Comte and goes to Milady's home, and finds himself seduced by her. The two have sex, and Milady--still believing d'Artagnan to be the Comte--gives d'Artagnan a ring, as a token of her affection.
In the sober light of the next morning, d'Artagnan realizes what a mess he's gotten himself into. He goes to Athos for advice. Athos tells him to stay away from Milady, and, oddly, recognizes the ring that she gave him as one he used to have. D'Artagnan writes Milady a letter as the Comte, saying that he must not see her anymore.
Broken-hearted Kitty faithfully delivers the letter, and Milady flies into a wild rage. She wants revenge on the Comte for spurning her, and so sends d'Artagnan a letter, inviting him to come visit her again. He goes, and she seduces him, exchanging sex for the promise that he will kill the Comte, whom she says has greatly insulted her. After a night of lovemaking, Milady presses d'Artagnan for details: how, specifically, does he plan to kill the Comte? D'Artagnan decides that enough is enough, and tells her that it was he she slept with as the Comte, and produces the ring to prove it. Milady is overcome with fury, and attacks d'Artagnan. In the scuffle, he tears her shirt, revealing that she has a Fleur-de-Lis branded on her left shoulder. Shocked and horrified, d'Artagnan escapes the dagger-wielding Milady, and runs out onto the street.
This section concludes the first half of The Three Musketeers. Lady de Winter, after spending the first half of the novel operating in the background, has become the book's most important antagonist.
Milady is considered by some to be Dumas’s finest literary creation. Beneath her aggression and cunning, at this point in the novel she is still a richly mysterious character. Is she Athos's murdered wife? If so, how did she survive? She claims to be an Englishwoman, yet she speaks perfect French; where is she from? What did she do to earn that Fleur-de-Lis? What is her connection to Madame Bonacieux's kidnapping, and to the Cardinal's schemes in general?
To a very great extent, Dumas has spent this closing portion of part I posing all these questions, in order to devote part II to answering them. In fact, one of the most common criticisms leveled against The Three Musketeers is that Milady dominates part II in a way that tampers with the novel's structural balance.
Milady indeed is the driving force behind the rest of the work. The power she holds over d'Artagnan ensures her ability to spur the musketeers into action. With each new revelation about her past, d'Artagnan promises himself more sternly that he will have nothing to do with her; but in her presence he is powerless. Milady possesses extraordinary powers of persuasion.
For his part, d'Artagnan has undergone some interesting changes, particularly in this last section of the story. He appears to be a collection of the virtues that suit one best to live well and successfully in his era. He is brave and loyal to his friends, and, when necessary, capable of intrigue. But in this section, he shows a ruthless streak unprecedented in the novel. His treatment of Kitty is almost openly cruel, and even his sexual deception of Milady is alarmingly devious. In part, of course, the extremity of his behavior is meant to indicate the extent of the effect Milady has had on him.
Still, this is a streak in d'Artagnan's character independent from his feelings for Milady: d'Artagnan is very, very ambitious. The moral universe of The Three Musketeers is an elegant one of high ideals, but those ideals live in close proximity to a kind of amoral sensuality which the novel seems to condone. Dumas’s characters are wholly devoted to their higher ideals, but they don't seem fettered by what one might consider "mundane morality." They're happy to die for each other or their King; in fact, they seem almost flippant about the thought; "death before dishonor" is their cheerful motto. But Porthos is happy to extort money from his Madame de Coquenard, Athos to drink the poor innkeeper nearly out of house and home, and d'Artagnan to break poor Kitty's heart. It seems that part of the charm of Dumas’s world is not only its high ideals, but also its lack of social constraints stemming from conventional morality. Romantic chivalry and Romantic amorality intermingle; in a way, his larger-than-life characters are above needing to worry about such things.