After leaving Bianchon, Eugène returns to the boardinghouse, where he finds a letter from Delphine de Nucingen inviting him to dinner and to the opera. His first reaction is that Delphine wants to make her unfaithful lover, Marsay, jealous, and he prepares to go out of curiosity to see what the girl's attitude will be.
He dresses very carefully, admiring himself in the mirror, and when he makes his appearance downstairs, he is greeted by the compliments of the bemused boarders and by Vautrin's satirical jokes.
At the Nucingens', Eugène finds Delphine completely downcast and making vain efforts to hide it. Taken a little aback by her attitude, Eugène urges Delphine to tell him what the matter is. Instead of answering his questions since he keeps on offering his help, she takes him to a gambling place, gives him a hundred francs, and asks him to bet them and try to win six thousand. A little uneasy at first, for he has never been in a gambling place and does not know what to do, Eugène becomes lucky and ends up winning seven thousand.
Delphine, overjoyed, undertakes to tell him why she needs the money so badly. Married to a man whom she did not really love, she pursued a separate life, using her savings and money she had borrowed from her father. Then she met Marsay and borrowed money from him, but when Marsay proved unfaithful, she wanted to return it. Her husband refused to help her unless she would be a wife to him. Now she can free herself from Marsay and give him his money back, thanks to Eugene's winnings.
The young couple have dinner together at Delphine's house and then leave for the opera, where their arrival creates a sensation among the Paris socialites, always in quest of new intrigues and love affairs. After the show, Delphine drives Eugène back within a few blocks of his boardinghouse. They separate until the next Monday, when Eugène is supposed to meet Delphine at the Duchess de Carigliano's ball.
Back at the boardinghouse, Eugène feels happy and disappointed at the same time; happy that his romance with Delphine is going so well, disappointed that he cannot use the husband to get into the financial world. Before going to bed, Eugène stops to talk with Old Goriot, who has been eagerly waiting for him, and tells him everything. The old man becomes very upset at Eugène's gambling and angry at his son-in-law, whom he threatens to sue. Eugène gives him the one thousand francs left from his winnings and asks Goriot to keep them for his daughter. Goriot is so touched by this that he cannot conceal a tear of gratitude, and Eugène, pleased with himself, goes to his room and falls asleep.
This whole section concerns Rastignac's further involvement with society, with Delphine, and therefore with Goriot. For as the young man gets more intimate with the young woman, he also gets closer to her father.
At the end of the section, in a touching scene when Eugène relates the incidents of the evening to Goriot, the old man bursts into a rage at his daughters' husbands, threatens to kill them, and with tears in his eyes at the young man's generosity says, "You will succeed. God is just, you see. I know an honest man when I see him, and I can tell you there are not many men like you. I am to have another dear child in you, am I? There, go to sleep."
At Delphine's house, Eugène is brought face to face with the immoral practices so common in this social circle. Delphine reveals the ugly details of her conjugal life and of her infidelity. She explains she has had to borrow money from her lover, how money is the essential instrument for women to attain social status and retain it, and how low they will stoop to acquire it: Half the women in Paris lead such lives as mine; they live in apparent luxury, and in their souls are tormented by anxiety. I know of poor creatures even more miserable than I. There are women who are driven to ask their trades people to make out false bills, women who rob their husbands. Some men believe that an Indian shawl worth a hundred louis only cost five hundred francs, others that a shawl costing five hundred francs, is worth a hundred louis. There are women, too, with narrow incomes who scrape and save and starve their children to pay for a dress. I am innocent of these base meannesses.
We notice also a further breach of the young man's code of ethics, for if he appears mildly shocked when Delphine asks him to gamble for her, his second reaction is the happy thought that "She has gone too far to draw back"; therefore, "she can refuse me nothing now!" Furthermore, Eugène learns a new, fashionable way of acquiring that almighty money, the source of social success. We will find him later trying his luck and oscillating between Delphine and society when he wins, and between Vautrin and his Machiavellian plan when he loses.