Julien and Mme. de Rênal's affair takes a downward turn when her youngest son falls deathly ill. Mme. de Rênal is convinced that God is punishing her for committing adultery and begs Julien to stay away from her. Julien thinks that her behavior is quite foolish, but worries that she might confess to both her husband and M. Chélan. She almost does, but then confides to Julien that she loves him more than anything else, even her own children. Moved by her devotion to him, Julien finally falls in love with Mme. de Rênal. Their renewed happiness tips off Mme. de Rênal's maid, Elisa, that something is going on and she tells M. Valenod, who writes an anonymous letter to M. de Rênal. Julien immediately recognizes that the letter denounces his affair with Mme. de Rênal. The two of them form a plan in order to convince M. de Rênal that the accusations are false and just an attempt to embarrass the mayor. They forge a second anonymous letter and pretend to send it to Mme. de Rênal. This second letter says that M. Valenod is both responsible for the first letter and out to seduce Mme. de Rênal himself.
Before receiving this second letter, M. de Rênal spends the entire night in a state of wretched embarrassment and hatred. More concerned about his name and political position than his marriage, he considers killing both Julien and his wife. But when Mme. de Rênal brings him the second letter, he no longer believes the accusations and thinks instead that M. Valenod is organizing a liberal plot against him. Mme. de Rênal manipulates her husband even further by showing him old love letters written to her by M. Valenod.
Julien is invited to dine with the Valenods, who want to hire him as the family tutor. He goes in order to give M. de Rênal the impression that all of M. Valenod's letters were intended to make Rênal fire Julien so he would move to the Valenod's. Julien is disgusted by the bourgeois liberals at the Valenod dinner party. They have all made money off the poor. Julien finds nothing honorable about their obsession with money and is doubly resolved to come to power through the Church. He gets his chance when the ever-jealous Elisa tells M. Chélan about the affair. Chélan, in order to avoid a scandal, arranges for Julien to enter a seminary in Besançon.
M. de Rênal, though now believing that nothing has happened between his wife and Julien, is happy to see Julien leave: the rumors will die out and M. Valenod will not get Julien to be his tutor. Mme. de Rênal is devastated to see Julien leave and gives him a lock of her hair. Although Julien is excited to become a powerful figure in the Church, along the road to Besançon, he keeps looking back at Verrières.
Stendhal's fascination with the psychology of love is very apparent in this section. It is only when Julien's vanity is sufficiently flattered by Mme. de Rênal that he falls in love with her. Stendhal describes this change as a switch from love of possession and beauty to passion-love. When Julien realizes that she loves him more than her own children, he trusts Mme. de Rênal's devotion enough to fall in love with her. Again, triangular desire enables Julien to love Mme. de Rênal (through the intermediary of her son). The love triangle between Julien, Mme. de Rênal, and Elisa also motivates the jealous Elisa to tell M. Valenod and then M. Chélan about Julien and Mme. de Rênal.
This section also juxtaposes Mme. de Rênal's intelligence and religious faith with M. de Rênal's foolish concerns for rank and lack of concern for his family. Mme. de Rênal truly believes that God is punishing her for falling in love with Julien. When she actually tries to confess to her husband, Julien admires her lack of hypocrisy. When it comes to saving her family's reputation, Mme. de Rênal also becomes very cunning. Julien realizes that M. Valenod has denounced them, but it is Mme. de Rênal who drafts the second letter and manipulates her husband into discovering M. Valenod's old love letters. M. de Rênal, in contrast, only cares about remaining the mayor of Verrières. He considers killing Mme. de Rênal and Julien but worries that then he would not receive an inheritance his wife is expecting. Mme. de Rênal easily manipulates her husband with flatteries into believing her version of the story.