After Rochefort leaves, Madame Bonacieux returns to Milady’s room to speak with her. Milady tells Madame Bonacieux that Rochefort is actually Milady’s brother who is only pretending to work on behalf of the cardinal. She also tells Madame Bonacieux that the letter she received from d’Artagnan is fake; he is not actually on his way, she says. Milady claims to have received this information from her brother. This information is worrying to Madame Bonacieux.
The story Milady tells Madame Bonacieux is similar to the one she told Felton; that is, it is completely fake. Milady knows Madame Bonacieux will believe her because she has no reason not to. Indeed, Madame Bonacieux is completely fooled and immediately put under Milady’s spell.
Pretending to be helpful, Milady tells Madame Bonacieux that she should leave along with her when her carriage arrives. Afraid and unsure of what to think, Madame Bonacieux agrees to go along with Milady’s plan. However, before Milady can carry out her plot, d’Artagnan and his friends arrive. Milady sees them first and tells Madame Bonacieux that they need to escape immediately because the cardinal’s guards are outside. This frightens Madame Bonacieux, who freezes up and doesn’t know what to do. In a last-ditch effort, Milady takes some poison, puts it in Madame Bonacieux’s glass, and tells her to drink it so that she will have the energy to escape. Not realizing what is in the glass, Madame Bonacieux drinks it without a second thought.
Because the musketeers ruin Milady’s plan, she has to think on the fly. Rather than overcomplicate the matter, Milady returns to one of her favorite methods of murder: poison. Because Madame Bonacieux trusts Milady, she drinks the poison willingly, ensuring that her reunion with d'Artagnan will not be a happy one.
D’Artagnan walks through the door and for a moment he is reunited with his love. However, after a few moments, Madame Bonacieux becomes weary, and she falls to the floor. She tells d’Artagnan and his friends about a woman she recently met who just left. To his horror, d’Artagnan comprehends what’s happened and knows that Madame Bonacieux is going to die. Moments later, his worst fear comes true.
Milady turns d’Artagnan’s moment of triumph into a scene of horror. In this pivotal moment, the novel reveals that it is not a typical adventure story; the heroes do not prevail, nor are they granted a happy ending. Instead, d’Artagnan is left to examine the failures that led to death of his beloved.
Shortly after Madame Bonacieux’s death, Lord de Winter arrives on the scene. They explain to him what happened and together all five men swear to track Milady down and seek vengeance. Although everyone agrees that they should go after Milady, Athos urges everyone to wait until the next day so that preparations can be made. No one is exactly sure what Athos means, but it is clear that he has something in mind.
Lord de Winter’s sudden arrival at the convent begins the novel’s steady rise toward its climax. Now that Madame Bonacieux is dead, all that is left is for the musketeers to take revenge.