"Soubrette" refers to a young and pretty lady’s maid—that would be Milady’s maid.
D’Artagnan continues to visit Milady every night and grows more deeply in love with her. Milady’s maid, Kitty, continues to brush past him after every visit. Finally, one night on his way in to see Milady Kitty accosts him and tells him that they need to talk.
They go to her room.
Kitty tells D’Artagnan that Milady doesn’t really love him.
D’Artagnan demands proof.
Kitty hands him a note addressed to the Comte de Wardes. He tears it open: in no uncertain terms, Milady is telling the Comte to court her.
D’Artagnan immediately wants revenge. He asks Kitty if she knows what it’s like to be in love, and whether she will help him.
Kitty points out that a) Milady will never love him, and b) she (Kitty) loves him. (She doesn’t say that last bit out loud, but it’s very definitely implied.)
D’Artagnan kisses her. He realizes that he has a lot to gain by her love. He can intercept Milady’s letters, for instance.
He tells Kitty that he loves her, and, as proof, he’s going to spend the evening with her instead of Milady.
Finally, midnight strikes and Milady calls for Kitty to help her get ready for bed.
Kitty tells D’Artagnan to leave, but he decides to hide in the closet. D’Artagnan stays there to listen to Milady and Kitty talking. First, Milady scolds Kitty. Then, she brags about how she has complete control over D’Artagnan. She complains that she hates him for making her look bad to the Cardinal and for sparing Lord de Winter’s life. Had Lord de Winter died, Milady would have been very rich indeed.
D’Artagnan listens and realizes that Milady is truly a monster.
Milady tells Kitty to try and get a response from Comte de Wardes this time.
Kitty returns and D’Artagnan comes out of the closet.
He kisses Kitty. (The narrator writes: "With a little more heart, he might have been contented with this new conquest; but the principal features of his character were ambition and pride.")
He wants to avenge himself of Milady.
D’Artagnan continues to visit Milady nightly, but he soon develops a plan.
Milady gives Kitty a third note for the Comte de Wardes. D’Artagnan makes Kitty promise to give him the letter. She does so. This time the letter is much more explicit. Milady declares her love for the Comte and says this is his absolute last chance to do anything about it.
Kitty worries that D’Artagnan is still in love with Milady, but he tries to allay those fears. He tells her that he wants revenge, that’s all. He forges a reply to Milady apologizing for not having written sooner and requesting a meeting at eleven in the evening. He signs it as the Comte de Wardes.
Although D’Artagnan understands that Milady is a monster, he still feels an incredible passion for her. He hands Kitty the note, who suspects that D’Artagnan doesn’t love her.
He promises Kitty that he will cut his visit short tomorrow in order to spend time with her.