Julien tries to divert his attention from Mathilde. He goes on a secret mission for the Marquis to help organize a conservative conspiracy that will strengthen the political power of the clergy in France. The Marquis, along with some of the most powerful men in France, wants to organize an army controlled by the Vatican. Julien's excellent powers of memorization impress the Marquis and his associates, so they send him around France to deliver their message to co- conspirators. Nevertheless, a group of conservative priests finds out about the conspiracy, forcing Julien to disguise himself as a soldier.
Upon his return to Paris, Julien decides to make Mathilde jealous. He begins writing copied love letters to an extremely religious member of the Marquis's salon, Mme. de Fervaques. Julien knows that she is too pious to even understand his fake declarations of love, and is just using her attention to upset Mathilde. He is still very much in love with Mathilde. For the two months that he forces himself to avoid her, he is miserable. During this period, Julien becomes an expert dresser and even manages to make Mme. de Fervaques begin to like him. Mathilde cannot help but notice Julien's surge in popularity, and blushes whenever they are in the same room.
Julien's ploy to make Mathilde jealous works perfectly. She finds out about the letters to Mme. de Fervaques and falls at Julien's feet proclaiming her undying love. Julien is madly in love with her too, but has learned how volatile Mathilde's emotions can be. He decides to say little and do little until he has guarantees from Mathilde that she will not change her mind. Julien now knows how to manipulate Mathilde's emotions, and succeeds in making her completely devoted to him.
They resume their relationship, but Julien is careful not too show too much emotion in order to keep Mathilde interested in him. However, she soon finds out that she is pregnant and wants Julien to become her husband. Mathilde immediately confesses everything to her father, blaming herself for seducing Julien in the first place. The Marquis is enraged but will not have Julien killed. He cannot imagine his daughter having the last name Sorel, and tries to think of some way to get rid of Julien. Instead, after a month of negotiations and the help of M. Pirard, the Marquis both gives Julien a large income and ennobles him as Julien de La Vernaye. Julien is also made a lieutenant in the army. Having made Julien a man of rank and wealth, the Marquis finally consents to Julien's marriage to Mathilde.
Julien's hypocrisy finally comes full circle as he finds himself working as an agent for a conservative conspiracy. Stendhal uses this political interlude to separate Julien and Mathilde as well as mock both liberal and conservative politics. Writing immediately following the bourgeois liberal Revolution of 1830, Stendhal replaces the liberal conspiracy with a conservative one, cynically suggesting that the two parties are really interchangeable. He further ridicules conservative politics when he introduces a second conservative conspiracy that attempts to thwart the first plot. Revealingly, nothing Julien does on his mission matters or has any effect. Just like during the king's visit to Verrières, Julien switches from the black clothes of the clergy to the red uniform of a soldier to avoid being recognized. Like liberal and conservative politics, the red and the black are really not that different from each other. Stendhal suggests that, like Julien's forgotten conspiracy, French politics are more comedy than drama.
Julien finally masters the psychological power of triangular desire in this section. The first part of his affair with Mathilde is hampered by her inability to decide whether she loves him or not. She especially cannot forget Julien's humble social status. However, Julien soon figures out that jealousy is the best way to win her devotion. Forming a love triangle with Mme. de Fervaques as an intermediary, he succeeds in making Mathilde completely fall in love with him and give him assurances of her devotion. Yet Julien is careful not to immediately declare his own love, making sure that he keeps Mathilde dependent on him and not the other way around. More so than ever, he evokes the example of Napoleon and treats her like an enemy on a battlefield that must be intimidated.