Frédéric becomes close to both Madame and Monsieur Arnoux and spends a great deal of time at their home. He earns Madame Arnoux’s trust, and she tells him about her background and about Arnoux’s changing careers. He is becoming vulgar, his business endeavors less honest. Frédéric advises her to leave Arnoux. When he is with Arnoux, he tries to convince him to leave Madame Arnoux. Frédéric likes Arnoux even though he is often irritating, but he is troubled to realize that Arnoux seems to be praising his wife more than usual.
Arnoux soon goes into heavy debt after an unwise business transaction, and his reputation suffers. Although others avoid the Arnouxes, Frédéric makes a point of seeing them as often as he always had. However, he begins to feel guilty about loving Arnoux’s wife. Arnoux, determined to avoid conflict, tells Madame Arnoux that Rosanette is not his mistress and that she is in fact Frédéric’s mistress. Furious, Frédéric tells Madame Arnoux it isn’t true. His position in the household is getting complex.
Deslauriers asks Frédéric for money to take over Hussonnet’s newspaper, and Frédéric agrees to give him the fifteen thousand francs he is expecting from his bank. However, Arnoux approaches him for a loan the next day, and Frédéric gives the money to him instead. He lies to Deslauriers about having not received the money. Arnoux is slower in paying it back than he promised, and when Deslauriers presses Frédéric, Frédéric says he lost the money gambling. Deslauriers declares their friendship dead.
Frédéric decides to write a history of the Renaissance. One day, Madame Arnoux comes to his room with her daughter, Marthe, and a maid. Madame Arnoux tells him that she and Arnoux owe money to Monsieur Dambreuse, and she asks Frédéric to talk to him to put off the repayment. Frédéric agrees, then they talk and walk in the garden; Frédéric gives her a rose.
Frédéric calls on Dambreuse, who agrees to wait to ask for repayment. He then offers Frédéric a job as general secretary in his coal company. Frédéric agrees to invest around forty thousand francs. He expects Madame Arnoux to be grateful for his help, but she sends only a brief reply.
On the day he is supposed to meet with Dambreuse about the job and investment, he instead chases after Madame Arnoux, who he has learned is at Arnoux’s factory in the country. It is less impressive than he’d been led to believe. She gives him a long tour, and he then hints to her about his feelings. She firmly rebuffs his advances. He returns to Paris, cursing her, and receives a note from Rosanette. He vows to go to her in revenge.