Before the Cardinal returns from his meeting with Milady, Athos goes on ahead, ostensibly to scout the way back. The Cardinal returns, and he and the other two Musketeers ride off toward the army camp.
Athos, meanwhile, has been hiding in the woods. He returns to the Inn where Milady is staying, and confronts her. She is shocked to see him, believing him to be dead. Athos declares his murderous hate for her, and warns her that, although he does not care about Buckingham, if anything happens to d'Artagnan he will kill her. To assure d'Artagnan's safety, Athos steals the written order of absolution that Milady got the Cardinal to give her--a message declaring that whatever the bearer of it has done has been done in the Cardinal's name. Athos leaves.
Athos returns to the camp with Aramis and Porthos. They immediately call d'Artagnan to see them, and go to eat breakfast at an Inn where they can talk quietly. The inn, however, is impossibly noisy, filled with soldiers. Athos makes a bet with a number of them that if he and his three friends can hold the Saint Gervais Fort by themselves for a full hour, the other soldiers must buy them a lavish dinner. Everyone agree to such a gutsy bet with great interest.
The friends have their breakfast packed up, and head out to the Fort. Athos explains that this deed will both bring them glory and provide them with a private place to plan what they must do, for anywhere else the Cardinal's agents would hear them. At the Fort, the men set up their breakfast, and raise a napkin to indicate that His Majesty's troops have occupied the fort. Athos then tells d'Artagnan and the others of his private meeting with Milady. They brainstorm on how best to keep themselves and Buckingham safe from Milady and the Cardinal. They decide to send two letters, both composed by Aramis. One will be addressed to Lord de Winter, warning him of Milady's plans on his life and criminal history, and urging him to imprison her when she gets to Britain. The other letter will be sent to Aramis's mysterious mistress (Madame de Chevreuse, the Queen's close friend) to warn the Queen of the plot against Buckingham. To get the money for these expeditions, the men decide to sell d'Artagnan's ring from the Queen.
During all of this deliberation, the four friends manage to fend off two attacking parties of Rochellese rebels, and stay in the fort a full hour-and-a-half. They return to great cheers, and are the celebrated heroes of the camp, so much so that news of their achievements comes to the Cardinal's ears. Convinced once again that he must have them on his side, the Cardinal congratulates Monsieur de Treville on his soldiers' bravery, and authorizes him to make d'Artagnan a Musketeer. Treville does so, and d'Artagnan finally joins their ranks.
The friends send the letter to Aramis's mistress with Bazin, and the letter to Lord de Winter with Planchet. D'Artagnan, who has a soft spot for Buckingham, also asks Planchet to give de Winter a verbal warning about Buckingham's assassination. Within a fortnight, both servants return with grateful replies, confirming the success of their missions.
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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