Frédéric and Rosanette settle into a cozy domestic life. Frédéric believes she looks different, but he cannot pinpoint why. He learns that Arnoux is being taken advantage of by a woman from his factory, that his factory has closed down, and that he and Madame Arnoux are nearly estranged. Arnoux visits Frédéric one day and asks him to visit their home again. As Frédéric makes his way there, he runs into a man named Compain, who asks him cryptically if he’s a member of the “calf’s head.” When he arrives, Arnoux is not home. He renews his love for Madame Arnoux, and they kiss. Rosanette, who has followed him there, sees them. At home, Frédéric is so angry at her intrusion that he nearly hits her, but he stops when she reveals that she’s pregnant. Frédéric helps her pay off a debt to Mademoiselle Vatnaz, which she claims is the last owed.
Frédéric becomes annoyed by everything Rosanette does and more and more enchanted with Madame Dambreuse. He is bored by the conversation that goes on at her parties, but he vows to make her his mistress. He visits her frequently and flatters her constantly. One day, she gives in, and Frédéric feels like a new man, finally a true part of society. As he walks home, he runs into Deslauriers, who has been embroiled in controversy and has been shunned by both the Conservatives and the Socialists. Frédéric invites him to stay with him, and Deslauriers encourages Frédéric to run once again for the Assembly. Later, Frédéric attends his first dinner alone with Madame Dambreuse.
When he returns home to Rosanette, he is secretly happy at his deception.
Deslauriers visits Dambreuse and begins working for him. He is in touch with Monsieur Roque, and Frédéric instructs him to lie to Roque and Louise about why Frédéric is delaying marriage. He becomes more infatuated with Madame Dambreuse when he sees her among guests, knowing that he is more intimate with her than anyone else, and society accepts their affair. Madame Dambreuse wants a grand passion from Frédéric, but he soon tires of her gifts and affections. He still wants to raise his position in society through his liaison with her, but he no longer feels the same attraction.
Dambreuse falls ill and dies. Madame Dambreuse is unmoved and happily tells Frédéric that she is the heir to a great fortune. He asks why Dambreuse didn’t leave his fortune to Cecile, who is not his niece but his illegitimate daughter, and Madame Dambreuse insults her endlessly. She asks Frédéric to marry her, and he agrees. He then arranges the entire funeral.
The day after the funeral, Frédéric visits Madame Dambreuse and finds her panic-stricken. She cannot find a will that declares that she is the heir; all she and her lawyer can find is a will naming Cecile as the heir. Frédéric claims he will still marry her and promises that he will love her forever. He tells her he is going to Nogent to prepare for his candidacy.