Athos sends the four servants out to Armentieres to scout Milady's exact location, while he and the rest of their party, now including Lord de Winter, attend Madame Bonacieux's funeral. Athos then embarks on a brief journey of his own--he seeks out a mysterious stranger who lives by himself, and convinces him to join their party, although the narrative does not tell us why, or what Athos says to the man.
Planchet then returns--the servants have found Milady, and the others are keeping an eye on her at the inn in Armentieres. With this news, Athos instructs everyone to get ready to ride, and goes to get the final member of their party. He returns with the mysterious stranger, a man in a red cloak, whom no one recognizes. The men set off after Milady.
The Musketeers, Lord de Winter, and the mysterious stranger find Milady as she is just about to cross over a river out of France. She is alone, and they capture her. They then try her. D'Artagnan brings his charges against Milady: the murder of Madame Bonacieux, attempting to murder him with the assassins and the poisoned wine, and inciting him to murder the Comte de la Fere. Lord de Winter brings his charges: the murder of his brother, and the murder of the Duke of Buckingham. At this the musketeers are shocked, for they had not heard of the Duke's assassination. Finally, Athos brings his charges, but just as he mentions the Fleur-de-Lis, Milady challenges them to find the court that branded her.
At this, the mysterious stranger steps forward. Milady recognizes him, in horror, as the Headsman of Lille. He completes Milady's story--she was a nun, and she seduced a young priest, the headsman's brother. They stole the Communion plate, and the priest was captured--but Milady escaped. The priest was branded; the Headsman himself had to brand his own brother. But the Headsman then hunted Milady down and branded her as well. After that, she escaped with the priest and entered Athos's territory, at which point Athos's story begins. The Headsman's accusation is this: Milady's robbery of the Communion plate, and the death of his brother, for the young priest went mad and hung himself after Milady abandoned him for Athos.
With the charges brought, Porthos and Aramis, acting as judges, sentence Milady to death for her crimes. The Headsman drags her outside, to do his duty. Milady tries frantically to stave off the inevitable--bribing the servants, reminding d'Artagnan of their love, claiming that the men have no right to kill her. Nothing works. The Headsman takes her across the river, ties her hands and feet, and cuts off Milady's head. He then takes her head and body and drops them into the river, to "God's justice."
The Musketeers now must return to duty at La Rochelle. Before they return, however, they run into Rochefort again, who arrests d'Artagnan in the Cardinal's name. D'Artagnan consents to the arrest, although his friends stay with him to protect him, and wait for him outside the Cardinal's quarters. For the second time, d'Artagnan is left alone with the great man. The Cardinal starts to tell d'Artagnan the crimes he's been accused of, but d'Artagnan cuts the Cardinal off, noting that the woman who brought these charges against him was a criminal herself, and is now dead. D'Artagnan then relates the entire story, from Milady's early history to her death, to the Cardinal. D'Artagnan then produces the Cardinal's letter of absolution, which Athos stole from Milady, which frees him from accountability for Milady's murder. For a moment, d'Artagnan's life hangs in the balance. The Cardinal could easily override the pardon, and have d'Artagnan executed. Instead, he gives d'Artagnan a promotion to lieutenant in the Musketeers with the name blank, and tells the young man to count himself as one of the Cardinal's friends.
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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