D'Artagnan wanders along the streets of Paris, lost in thoughts of love for Madame Bonacieux. He decides to visit Aramis, and is surprised to find a young woman knocking on his friend's door. He is further shocked when he sees that the young woman's knocks are not answered by Aramis, but by another woman. The two women exchange handkerchiefs, and Aramis's visitor leaves. D'Artagnan's final shock comes when he sees that the visitor is Madame Bonacieux.
D'Artagnan follows Madame Bonacieux and asks what she was doing. After getting over some shock, Bonacieux seems charmed by the fact that d'Artagnan wants to protect her, and allows him to escort her to the next house on her secret mission. She then makes him promise not to follow her anymore, and he very reluctantly agrees.
D'Artagnan returns home to another surprise: Athos has been arrested, because the police thought he was d'Artagnan. D'Artagnan heads immediately to the Louvre to talk to M. de Treville about this. Along the way, he again spots Bonacieux, this time being escorted by Aramis. Angry that she lied to him and that his friend is betraying him, he confronts the couple, to find out that the man is not Aramis, but the Duke of Buckingham himself, off to a secret tryst with the Queen. D'Artagnan guards their passage into the Louvre, and goes to make sense of the day's events.
At the Louvre, the Duke of Buckingham and the Queen have a very emotional meeting. The duke knew that it was a Cardinalist trap that brought him to France, not the Queen's summons, but he still had to see her. He professes his undying love, but she is cautious; she clearly loves him, but feels duty-bound to be distant. Buckingham says that he will wage war on France, killing thousands, just to have an excuse to be near her. Eventually, he gets her to give him a token of her esteem: a diamond brooch that Louis XIII had given her for her birthday. With that, he is gone with a flourish, leaving the Queen in a state of confused emotion.
Meanwhile, Monsieur Bonacieux has been held in the Bastille, and he is horrified. After being interrogated by two minor magistrates, he is brought into the presence of Cardinal Richelieu himself. Cowed by the great man, Monsieur Bonacieux tells the Cardinal all about his wife's activities, and promises to keep an eye on her in the future. The Cardinal has convinced the little landlord to spy on his own wife for him.
The next day, M. de Treville finds out about Athos's arrest, and immediately goes to the King to get him released. However, the Cardinal arrives before de Treville, and gets a chance to convince the King of the merit of his side of the story. Monsieur de Treville, however, is able to convince the King that it is ridiculous to arrest one of the musketeers without cause. When the Cardinal questions M. de Treville about d'Artagnan, de Treville is able to respond honestly that d'Artagnan was at his house at the time of the arrest--d'Artagnan had reset the clocks when he visited de Treville the night before, to assure that his alibi would be secure. The Cardinal then backs off, and allows the King to free Athos. M. de Treville leaves, happy but suspicious about the Cardinal's sudden change.
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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