D'Artagnan arrives at the inn where he left the wounded Aramis, and discovers him and two churchmen discussing Aramis's religious thesis: Aramis has again decided to join the Church. D'Artagnan figures out the root of Aramis's conversion, however--Aramis believes that his mysterious mistress, the author of the letter d'Artagnan picked up, has abandoned him. D'Artagnan produces the letter and, upon reading it, Aramis's whole attitude changes--he no longer wants to renounce the world; in fact, he seems more enthusiastic about living life than ever. By now, D'Artagnan has figured out that Madame de Chevreuse, the mysterious woman who met Madame Bonacieux in Aramis's apartment at the beginning of the story, is in fact Aramis's mistress. Aramis needs more time to recuperate from his wound, so d'Artagnan leaves him to go in search of Athos.
At the inn where he and Athos were ambushed, d'Artagnan is harsh with the innkeeper, who was part of the attack. It becomes clear, however, that the innkeeper was a pawn in another Cardinalist plot to capture d'Artagnan. After d'Artagnan left, Athos fought off his attackers, and barricaded himself in the inn's basement with Grimaud, where he has remained since then. This is causing the innkeeper considerable consternation, as all his wine and food is stored there, and he can't get in. D'Artagnan coaxes Athos out. He and Grimaud emerge reelingly drunk; they have deliberately been trying to consume all the landlord's stock to punish him for the attack. This causes everyone except the landlord considerable amusement, and Athos and d'Artagnan take a regular room in the inn.
In his drunken stupor, Athos makes an extraordinary confession to d'Artagnan. He tells of a "friend" of his, a nobleman, who married a young woman of humble background under his rule, breaking the rules of social conduct for idealistic love. One day, his friend discovered that this woman was branded with the Fleur-de-Lis on her left shoulder, a symbol put on the most heinous of criminals. The young woman was a fraud; all she wanted was money and social power. In mad sadness, Athos's friend hung his wife. After hearing this horrible and extraordinary story, which is clearly about Athos himself (Athos lapses into the first person toward the end of it), d'Artagnan feigns sleep, unable to take any more from his friend that night.
The next day, Athos and d'Artagnan both agree to chalk up the previous night's conversation as drunken rambling on Athos's part, and not mention it anymore. They then head back to Paris, picking up their friends along the way. Upon returning to Paris, each of the four friends finds a letter from M. de Treville informing the musketeers that the campaign against Britain has begun, and that they must equip themselves. This causes considerable consternation, as they simply don't have enough money to outfit themselves appropriately. D'Artagnan is also immediately interested in knowing if there has been any news of Madame Bonacieux in his absence, which there has not.
The first of the friends to solve his equipment problem is Porthos. He is able to get back into his mistress's (the attorney's wife, Madame Coquenard) affections, and she supplies the funds. D'Artagnan happens to witness the beginning of this scene, but becomes extremely distracted when he spots the Woman from Meung. He feels that this woman is somehow affecting his life, and he is right--she is Milady, an agent of the Cardinal.
D'Artagnan becomes obsessed with Milady, her connection to the Man from Meung, and how it all relates to Madame Bonacieux's disappearance. He happens to run into her having an argument with a man on the street, and seizes the opportunity to intercede. A fight breaks out between d'Artagnan and the man, Lord de Winter, who turns out to be Milady's brother. The two men make an assignation to duel the following day.
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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