Lord de Winter sends Felton away, suspecting that Milady has won him over. However, the day before Milady is to be banished, Felton returns to rescue her. He breaks her out of her prison, and they escape. Felton explains that his plan is to kill Buckingham, and then go to France with Milady. Milady is delighted at the success of her manipulation.
Felton and Milady land their ship at Portsmouth, where Buckingham is preparing the British navy to set out for La Rochelle. Felton goes to the Duke's headquarters, and gains admittance by claiming to have an urgent message from Lord de Winter. Felton first tries to convince Buckingham to sign Milady's release; when Buckingham refuses, Felton stabs him, fatally. Before Buckingham dies, a messenger arrives with a letter from the Queen, begging Buckingham to call off his war against France, which the Queen knows he is waging out of love for her. The messenger also has a verbal message from the Queen for Buckingham: she still loves him. The Duke dies a happy man.
At La Rochelle, the Musketeers are extremely anxious to go and get Madame Bonacieux, as they now know that she is being kept at the same convent that Milady is supposed to go to if she returns from England. Sensing their distress, Monsieur de Treville gives them five days of leave to do what they need.
Immediately, the four friends set off to get Madame Bonacieux. Along the way, they have another chance encounter with the Man from Meung, who again escapes from d'Artagnan. He drops a note, however, on which is scrawled the name of a town: Armentieres.
Meanwhile, Milady has beaten the musketeers to the convent. She befriends Madame Bonacieux, under the guise of being a good friend of d'Artagnan's who is also being persecuted by the Cardinal. The innocent Madame Bonacieux trusts Milady wholly, and goes so far as to show Milady a letter from Madame de Chevreuse notifying her that d'Artagnan is coming to get her. Milady considers all of this terrific luck, for now she can use Bonacieux to get to d'Artagnan.
The Man from Meung then comes to visit Milady. He is the Comte de Rochefort, the Cardinal's personal spy. Milady arranges with him to have a carriage come for her and Madame Bonacieux as soon as possible, to take them to Armentieres, which she writes on a note for him. Milady then tells Bonacieux that Cardinalist agents are coming to get her, and the only way she can escape them is to come with her when she escapes the convent shortly. Bonacieux, trusting as ever, prepares to leave with Milady.
Rochefort's carriage arrives, and the two women prepare to leave. Just then, d'Artagnan and the Musketeers come riding up, but Milady tells Bonacieux that the Cardinalist agents have come to get her. Bonacieux trusts Milady enough not to look out the window for herself. Milady tries to hurry Madame Bonacieux into her carriage, but the young woman is too frightened to move. Disgusted, Milady poisons a glass of wine, gives it to Madame Bonacieux to "give her strength," and escapes alone.
D'Artagnan bursts in just as Milady's poison is starting to take effect. Bonacieux explains that her "good friend" Lady de Winter has just left. The men realize what has happened, and are horrified as Madame Bonacieux dies in d'Artagnan's arms. Just then, Lord de Winter himself arrives from England, in pursuit of Milady. Athos reveals his identity as Milady's first husband to de Winter, and his friends, and the men agree to rest briefly, then head out after the murderess.
For most of his novel, Dumas has taken care to keep his fictional characters out of the direct path of major historical development; this section is the main exception. Dumas takes it upon himself to involve Milady intimately with the assassination of the Duke of Buckingham, and seems quite serious about the historicity of his version: the chapter in which Felton kills Buckingham is entitled "What Happened at Portsmouth on 25 August 1628." Dumas is again translating history into Romance, and rewriting the details; one might almost take The Three Musketeers as an extremely entertaining speculation on the stories behind history, which can never really be known.
Historically, George Villiers, First Duke of Buckingham, was in fact assassinated by a naval lieutenant named John Felton, whose motives seem to have been both personal and political. But Milady is a completely fictional creation of Dumas; she did not exist to effect the assassination. Dumas’s unique formula is again evident: he makes an important historical event, the assassination of Buckingham, pivotal to his story. His facts about the assassination are all accurate. He then creates a story surrounding this event, based on this historical event, but having little to do with history itself.
Madame Bonacieux's death comes at an alarming time for d'Artagnan and his friends. Throughout the novel, one of d'Artagnan's major drives has been his love for Madame Bonacieux and his desire to get her back. The unhappy ending of her death seems out of place in the sort of story that Dumas started out writing, the light-hearted adventure. With Madame Bonacieux's death, the darkening of The Three Musketeers is nearly complete. When Athos sees the poisoned wine and guesses what has happened, he says "God wouldn't allow that; it's too vile!"
How vile has Dumas’s world become, then? What has happened to the confidence and cockiness that suffused his world at the story's start? Dumas’s swashbuckling adventure does not end happily; love does not prevail. What happened? Dumas is beginning to develop the more melancholy aspects of the Romance, which are common to the genre. Romance often lends itself to maudlin tragedy; Buckingham's death, in fact, is almost archetypically Romantic: fatally wounded, all he wants is news of Anne and, upon hearing that she loves him, he dies happily.
But Dumas is subjecting his readership to interesting ambiguities by raising the question of how far the chivalric code will take our heroes in their quest for bloody revenge. D'Artagnan is not interested in melancholy, or, in some ways, in Romance itself: he is not content to mourn Madame Bonacieux passively. He will overcome his sadness by achieving justice through revenge.