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 accordance to that fact that Athos is always melancholy, we actually do know the source. During the book, while Athos and d'Artagnan get drunk, we discover Athos had an ex-wife, which turns out to be Milady. Later, we find that Athos did not know that Milady was a criminal when he fell in love with her. When he learns of her past, he believes that he hangs her.
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