The two boarders Poiret and Madamoiselle Michonneau meet with a police detective named Gondureau. Gondureau is part of an investigation tracking an escaped criminal known by the nickname of "Death-Dodger." Police have become increasingly convinced that Vautrin is the Death-Dodger, and hope to arrest him for his work trafficking money through a large criminal network. However, they want to be absolutely certain that they are arresting the right man, so they are hoping the fellow boarders will agree to drug Vautrin and then identify him by looking for a branding mark while he is unconscious. They are willing to pay the boarders to do so.
Madamoiselle Michonneau asks for additional time to think about it, and Gondureau tells her to be in touch once she has made her decision. As the group disbands, Bianchon overhears part of their conversation. Once the police officer has left, Michonneau explains that she wants to find out if she can get even more money by cutting a deal with Vautrin himself.
As they arrive back, they see Rastignac and Victorine speaking tenderly with each other. Although he still feels guilty, Rastignac has started to woo Victorine. Vautrin also comes back and explains that his friend and Victorine's brother have quarreled, and a duel is arranged for the following day. Tormented by guilt, Rastignac decides to try and warn the Taillefer family, but he is interrupted by Goriot, who is eager to explain that he has secured an apartment for Delphine and Rastignac to live in together. The old man hopes he will be permitted to live upstairs and be closer to his daughter. Rastignac agrees to go and see Delphine that evening, but asks Goriot to try and stop the Taillefer duel. That night, however, Vautrin conspires to get everyone very drunk in order to prevent anyone from being warned about the duel. Both Goriot and Rastignac pass out early at the boarding house without going anywhere.
However, Vautrin does not realize he has sealed his own doom. The drunken distraction prevents Bianchon from mentioning what he overheard, and Mlle. Michonneau decides to betray Vautrin. She goes to visit the police officer and returns with the drug. The next morning, she drugs Vautrin's coffee just as all the boarders assemble for a late breakfast. Rastignac is dismayed to receive a note of rebuke from Delphine, who is disappointed he did not visit her the previous evening. Before he can react, word arrives that Victorine's brother has been gravely wounded in a duel, and she and Madame Couture hurry to her father's house.
Vautrin collapses from the drugged coffee, and in the ensuing chaos, Mlle. Michonneau confirms that he is the Death-Dodger by viewing the brand on his shoulder. Rastignac vows that he will remain faithful to Delphine, even with Victorine's new wealth. When he returns, Vautrin is mostly recovered, and seems to think Rastignac was responsible for poisoning him, but it quickly becomes clear that Mlle. Michonneau was the one to betray him. Before Vautrin can lash out, he is seized by the police and arrested. After he is taken away, the other boarders band together and demand that Mlle. Michonneau be thrown out of the boarding house. She leaves, accompanied by Poiret, and shortly thereafter, news comes that Victorine's brother has died, and that she and Madame Couture will not be returning.
This news is followed by Goriot's arrival, in an urgent hurry to bring Rastignac to Delphine. In a daze, Rastignac is taken to a beautiful apartment, where he is greeted warmly by Delphine and offered money from Goriot. Later that night, Goriot and Rastignac announce to a devastated Madame Vauquer that they will be moving out of the boarding house.
Rastignac receives an invitation from Madame de Beauseant inviting him and Delphine to her upcoming ball. Delphine notes that her sister will also be attending the ball since vicious rumors have been circulating that Anastasie has pawned her diamonds to pay off her lover's debts. Anastasie will want to be seen wearing the diamonds in public in order to put these rumors to rest. For the moment, it seems that Rastignac, Goriot, and Delphine face a bright and happy future.
Rastignac has been torn between his loyalty to Goriot and Delphine on one hand, and his temptation to profit from Vautrin's scheme on the other. Although he never fully commits, his pressing financial difficulties make it harder and harder for him to resist Vautrin's plans. Victorine, like many of the other female characters in the novel, seems trusting and led by her emotions. She does not question anything when Rastignac begins to show an interest in her. Vautrin is also crafty enough to protect his scheme, knowing that Rastignac will likely have a last-minute change of heart. By using drugged wine, Vautrin prevents Rastignac from delivering his warning, and the scheme does come to fruition.
Victorine ends up being effectively the only character in the novel where virtue is rewarded, since she ends up being reconciled with her father and elevated to wealth. However, nothing is shown of her fate after she moves out of the boardinghouse. Given what readers see of Delphine and Anastasie, there is no guarantee that Victorine will end up happier in her new life.
Vautrin has been scheming without realizing that he is also the target of another scheme. Police shrewdly decide to use other boardinghouse residents in their effort to arrest Vautrin. Both Vautrin and readers have likely paid very little attention to Mlle. Michonneau because she seems so harmless and dull. Her ordinariness gives her the perfect disguise to conspire against Vautrin, and he does not realize until the last moment that she has been the one to betray him. More strikingly still, Mlle. Michonneau is revealed to be as greedy and ambitious as any of the other characters. She is motivated by financial gain, not justice or ethics, and in fact would be willing to help Vautrin escape if that action would earn her a bigger profit. When the other boarders react to her betrayal with disgust, they seem to consider her as more morally dubious than Vautrin himself. While Vautrin has lied and schemed, he did not violate the sense of community within the boarding house.
Vautrin's capture is an interesting conclusion for a character who is extremely successful at hiding his true identity and seemed on the verge of getting away with past crimes. His arrest is an oddly traditional choice in which the villain figure is appropriately punished, and the conflict neatly resolved. Although Balzac's plot continues past this point, the arrest does offer a seemingly tidy resolution to at least one of the main conflicts in the novel.
Vautrin's arrest seems to neatly resolve Rastignac's dilemma. He chooses to recommit to Delphine, passing up the opportunity to try and marry Victorine. Through this choice, Rastignac violates the advice he has been given to be calculating and pragmatic. His desire for Delphine, and his affection and loyalty to Goriot, lead him to make a riskier choice. However, it seems to pay off in the short term when he is established in a lovely apartment and finally able to consummate his relationship with Delphine. Goriot functions as a sort of fairy godmother here, eagerly bringing together his loyal friend and his daughter. The chance to unite them allows Goriot to create a surrogate family unit where he can finally cultivate the close relationship he has always wanted.