Julien is shocked one morning to see Mathilde dressed all in black. He tries to figure out who she is mourning, and learns that it is the anniversary of her famous ancestor's death. Her ancestor, Boniface de la Mole, was decapitated for leading an insurrection in 1574. His lover, Queen Margot, asked for his head and buried it. Impressed by this romantic tale, Julien spends more and more time with Mathilde and soon begins to confide in her. They find that they both admire Napoleon and a time when men were heroes. Mathilde is an admired beauty in high demand by Parisian society, so Julien decides to seduce her. But Mathilde has already fallen in love with him. Unlike the noblemen who declare their love to her everyday, she finds Julien exciting. More importantly, she is extremely attracted to the bold notion of a forbidden love between an aristocratic woman and a man so far below her on the social scale. Mathilde also finds his fiery ambitions and liberal political aspirations a welcome change from the boring nobles she is used to. Even before Julien forms a plan of seduction, Mathilde declares her love.
Julien feels he has triumphed over the countless noblemen who have tried to seduce Mathilde, but worries that her declaration might be an elaborate trap to humiliate him. Mathilde is so impressed with Julien's precautions to come to her room (using a ladder and armed with a pistol) at one in the morning that she calls him her master and husband. They become lovers, but Mathilde feels less passion than a sense of duty toward Julien.
The next day, Mathilde worries that Julien now holds too much power over her. They begin to argue, and Julien becomes so enraged that he threatens her with one of the Marquis's antique swords. Mathilde is so moved by the knight-like ferocity of Julien's gesture that she feels transported back to a time when jealous husbands often killed their wives. The sheer intensity of their relationship is too much for both of them and they begin quarreling again, only to rekindle their passion at the opera. Julien returns to her window with a ladder and Mathilde chops off half of her hair to give to Julien as a sign of her obedience. But she soon renounces her love for him again. Julien tries to forget her, but realizes that he is hopelessly in love.
Mathilde suffers from an acute case of Parisian boredom and, like Julien, yearns for a more adventurous epoch. Rather than looking for excitement in the recent past (as Julien does with Napoleon), Mathilde admires the romantic chivalry of the sixteenth century. She knows its history very well, especially that of her ancestor Boniface. Contemporary readers of Stendhal would also know the story of Boniface de la Mole and Queen Margot's love affair from Alexandre Dumas's Queen Margot. The plot of Dumas's novel mirrors the second half of The Red and the Black. This technique allows Stendhal to foreshadow Julien's fate as well as evoke the boredom of the nineteenth century: adventure is only experienced in books.
Again, triangular desire plays a major role in Mathilde and Julien's relationship. Mathilde is looking for a modern day Boniface to sweep her off her feet. Julien's lack of exposure to Parisian society and fierce ambition make him stand out among the boring members of the salon. Mathilde often laments her "dull degenerate century" and thinks that Julien would make a great general in sixteenth-century France. She soon begins to imagine that Julien is a romantic throwback to a more exciting time. She thus loves Julien through the intermediary of the Boniface legend, forming yet another love triangle.
Julien's love for Mathilde is tempered by two factors. Stendhal set out to write a novel about a France divided by class; Julien can not think of Mathilde without being aware of the wide social gulf between them. He first thinks that she is an arrogant snob and wants to seduce her just for the challenge. But when Mathilde breaks with tradition and declares her love first, Julien cannot help but think that it is some elaborate form of ridicule. His extreme caution and daring actually end up making Mathilde love him even more. As usual, he treats their secret meeting like a general planning an attack: after making sure that no one is following him, he climbs up to her window with a ladder, pistol in hand. Julien really thinks that he is risking his life, and Mathilde loves him for it.