After the death of Buckingham, the king of England closes all the ports, but Milady has already escaped, and one other ship also left. Dumas comments cryptically, "We will later see who was aboard it and how it left."
In France, everyone — including the king — is bored with the siege of La Rochelle. The musketeers, meanwhile, receive a letter from Aramis's ladyfriend, Madame de Chevreuse, with a note that gives freedom to Constance Bonacieux and puts her in their care; the note is signed "Anne."
The king, being bored, needs an escort to Paris, and the four musketeers are among those chosen. In Paris, because they have no pressing duties, they obtain a leave of absence so that they can go to the convent. Once there, d'Artagnan again sees his nemesis, "the man from Meung." As the man rides away, he drops a piece of paper which d'Artagnan retrieves from his servant; on it is written one word, "Armentiers."
Meantime, Milady has also wended her way toward the convent, where she is received as a gracious lady since she has the blessing of the cardinal. However, she senses that the Abbess is not a cardinalist, so she pretends to be a victim of the cardinal (instead of a friend of the cardinal), thereby hoping to gain favor with the Abbess. By chance she learns that another "persecuted person" is at the convent, a woman named Kitty. Milady is anxious to meet this "Kitty," because her own maid Kitty helped d'Artagnan deceive her.
After a brief nap, Milady awakens to discover a beautiful novice standing at her bedside. After they talk, they discover that they are both victims of the cardinal's persecution; Milady, of course, is lying, but she shrewdly questions the other woman and, to her astonishment, she realizes that she is talking to Constance Bonacieux — the very woman she wants killed. Immediately, she ingratiates herself into Constance's confidence, and Constance innocently shares a letter from Madame de Chevreuse. The letter says that d'Artagnan will be arriving for her very soon. At that moment, a man on horseback arrives, asking for "a lady who just came from Boulogne."
The visitor is Count de Rochefort, the cardinal's right-hand man, the person whom d'Artagnan always refers to as "the man from Meung." Milady immediately tells him three important matters: (1) Buckingham is either dead or seriously wounded; (2) she has become a close confidante to Constance Bonacieux, whom the cardinal is searching for; and (3) the four musketeers will be arriving soon. The count is to report immediately to the cardinal, but Milady asks that he leave his chaise, his servant, and his money at her disposal. He is also to instruct the Abbess that Milady is to be allowed to walk in the woods.
Milady then plans her revenge. She pretends that the visitor was her brother, and she convinces Constance Bonacieux that the letter from Madame de Chevreuse is a forgery and that they must flee to a secret cottage that Milady knows about. Suddenly they hear hoofbeats and, leaving Constance seated, Milady peers through the window and sees the musketeers approaching. She tells Constance that it is the cardinal's men and they must escape through the woods. Constance is so paralyzed with fear that she can't move, so Milady pours her a glass of wine, while opening a secret ring and pouring poison into the wine. Then she makes the innocent and trusting Constance drink the wine. Afterward, she flees for her life.
D'Artagnan and the others arrive and find Constance weak and dying. With her last bit of strength, Constance embraces d'Artagnan and tells him that she loves him. As he holds her tenderly in his arms, she is able to remember the name of the woman who gave her the poison. Then she dies, and d'Artagnan "now held only a corpse in his arms." At this moment, de Winter enters, identifies himself and explains that he has been following close behind Milady ever since the death of Buckingham.
D'Artagnan is prostrate with grief, but is comforted by Athos, who tells him to "weep, weep, young heart filled with love, youth, and life!" The other musketeers want to take revenge against Milady immediately, but Athos insists that he be in charge because "she is my wife."
The king's boredom allows the musketeers the opportunity to accompany him to Paris and, from there, to continue to the convent with the letter from Queen Anne authorizing the release of Constance Bonacieux.
In a romantic novel such as this one, coincidences often play a large part in the plot. Thus, the piece of paper with only the name of a town, "Armentiers," written on it, proves to be a very valuable find because the musketeers feel sure that this is the town where they will be able to find Milady.
At the convent, we again witness Milady's knowledge of psychology and her ability to win the confidence of such different people as the Abbess and Constance Bonacieux. We also see additional proof that Milady is corrupt to the core; she does not even know the young and innocent Constance Bonacieux, but she fiercely desires her death — in order to get even with d'Artagnan. A woman who would sacrifice the life of an innocent victim only to satisfy her own selfish lust for revenge deserves the worst punishment available.
The death of Constance Bonacieux at the very moment that her supposed savior, d'Artagnan, arrives is typical of nineteenth-century melodramatic romanticism. This scene is one that still affects most readers-in spite of its overt use of sentimentalism and contrived timing. The death of this young woman causes Athos, who earlier had been so secretive about his past, to reveal that the scheming, vicious Milady is his wife and that he will take personal charge of punishing her.