The first thing D’Artagnan says when he walks into Madame Bonacieux’s apartment is that her husband is a bad guy. He overheard the entire conversation, and he’s ready to do anything for her.
Madame Bonacieux says, "Really?"
D’Artagnan claims that he’s a trustworthy guy. He ends with "I love you. I’m an honorable man. I am gallant. I am brave."
Madame Bonacieux looks at his eager puppy-dog face and decides to share the Queen’s secret. She also, the narrator says, is kind of falling for him. She tells him the whole story—the narrator calls it "their mutual declaration of love."
Before he leaves, Madame Bonacieux points out that he can’t just take off. He needs to ask Tréville boss for a leave of absence. She also points out that he needs money. She gives him her husband’s money. (The money from the Cardinal!)
D’Artagnan remarks that it will be amusing to save the Queen with the Cardinal’s money.
There are more declarations of love.
Then Madame Bonacieux notices that her husband is talking to someone in the street. The two of them need to do the sensible thing, which is to hide in D’Artagnan’s apartment.
Once in D’Artagnan’s apartment (which is right upstairs from Bonacieux’s, remember?) the two lovebirds look out the window and see Monsieur Bonacieux chit-chatting with D’Artagnan’s worst enemy, the Man from Meung!
D’Artagnan wants to run off and slay him right then and there, but Madame Bonacieux orders him not to.
Because he’s madly in love with Madame Bonacieux, D’Artagnan stops puts his sword away.
The two of them eavesdrop on the conversation downstairs. The stranger asks Monsieur Bonacieux if his wife suspects anything. Bonacieux says no.
The stranger asks Bonacieux to make sure that D’Artagnan isn’t home. Bonacieux does so as the two lovers sit quietly.
The conversation continues—Bonacieux is certain that his wife is at the Louvre, and Bonacieux did not hear his wife mention any names.
The stranger then scolds Bonacieux for not pretending to accept the mission so they could have the letter.
Bonacieux responds, "my wife adores me, and I’ll just run over to the Louvre and tell her I changed my mind about the mission!"
Madame Bonacieux curses both her husband and the stranger under her breath.
The stranger leaves and Bonacieux discovers the missing money. He runs screaming out of his house.
D’Artagnan asks if he would win Madame Bonacieux’s affection with a successful mission. She blushes, which means "yes," and shortly thereafter, D’Artagnan departs.