Julien is accompanied to Paris by two liberal landowners berating the provinces for their tedium; the scene is thus set for this exciting new chapter in Julien's life. After a brief detour to Malmasion, Napoleon's Paris home, Julien is greeted by the abbé Pirard, who prepares him to meet his new boss. Julien reciprocates with filial gratitude. Nonetheless, Julien fails to avoid the comic faux- pas of a country boy in the big city for the first time; he bows to his tailor and commits the most basic orthographic errors ('cella' instead of 'cela'). The Marquis has two children, Norbert, who takes Julien riding to disastrous effect, and Mathilde, a spirited girl with a healthy disregard for the social conventions of her station and an illicit love of slightly risqué literature. As a member of the household, Julien is expected to dine with the family and attend their salon in the evening; Stendhal paints a detailed portrait of the excruciating boredom of Restoration society, a boredom that certainly does not escape his protagonist, inevitably on its fringes and often mocked by Norbert and his friends. Nonetheless, the Marquis soon comes to appreciate his hard work.
An incident in a café wounds Julien's pride and after a series of misunderstandings he finds himself engaged in a duel with the chevalier de Beauvoisis, who, for the sake of his own reputation, propagates the rumour that Julien is in fact of noble birth. This idea amuses the Marquis, who indulges it by buying Julien a second outfit; in his new blue suit the Marquis treats him as an equal. He even sends him on business to England - on his return he finds the obsequious Valenod (now a baron) trying to wangle an introduction to Julien's illustrious new circle. Indeed Julien's social circle is constantly expanding; he attends a sumptuous ball given by the Duc de Retz, where Mathilde is much admired and where, in the context of so much extravagant frivolity, he discusses his politics earnestly with the Spanish Count Altamira. The Count is in exile having been condemned to death in Spain, a proof of heroism that earns him Julien's undying respect. Julien's singular pride has also earned him more than respect from Mademoiselle de la Mole, who begins to show her father's secretary distinct signs of devotion (she too ominously pays her own homage to romantic heroism by wearing mourning to honour the memory of an executed ancestor). Stendhal proceeds to explore in greater detail the nature of her feelings, a strange compound of attraction and rebellion, often in the first person. Given the social distance between them Julien is very wary of her attentions, believing that he may be the victim of an elaborate plot to ridicule him. On hearing that Julien is going away on business Mathilde however begs him to stay and declares her love. Her declaration leaves Julien feeling invincible although not without niggling anxieties. Mathilde solicits an illicit rendezvous for that evening.
Julien deliberates at great length but finally resolves to climb a ladder to her bedroom in the early hours of the morning; at least she will never be able to accuse him of cowardice. An awkward scene ensues but both characters fulfil the roles they have assigned themselves; Julien seduces Mathilde. Immediately after the event they both struggle to come to terms with their vanity; Mathilde is horrified by the idea that she has voluntarily enslaved herself and Julien is embittered by her coldness to the extent that he seizes an ornamental sword and almost stabs her. Thrilled by such a display of uncontrolled emotion, Mathilde renews her declaration, but Julien makes the fatal mistake of revealing the full extent of his devotion, reversing their positions in this fraught war of attrition. A visit to the opera restores Mathilde's passion and Julien pays her a second (and more successful!) nocturnal visit. Should the couple's behaviour begin to appear implausible the author intervenes, comparing the novel to a mirror - it is not he who is at fault but society. Despite cutting off her hair as a token of her love, Mathilde's haughty pride nonetheless resurfaces and Julien is thrown from the height of bliss to the depths of humiliation. The couple argue violently.
Meanwhile the Marquis is embroiled in an Ultra (extreme Royalist) plot to effect a full restoration of old regime values; other characters involved include a M. de Nerval (clearly modelled on Charles X's chief minister Polignac). The Marquis puts Julien's extraordinary memory to the service of the conspirators and sends him on a mission to Strasbourg to repeat verbatim the discussions of a secret meeting. In Strasbourg he renews his acquaintance with Prince Korasoff whom he had first met in London. Disguising names, he confides the problems of his relationship with Mathilde to the Prince who proposes a solution: he should pretend to court another woman. Julien immediately thinks of the prudish Maréchale de Fervaques and Korasoff obligingly provides him with a set of 53 letters to copy out and send. After seeking further advice from Altamira Julien embarks on his plan on his return to Paris, Mathilde - who in his absence has begun to renew her courtship with the illustrious Marquis de Croisenois - is incensed. Julien initiates a correspondence with the Maréchale, who initially seems unmoved; however, she invites him to dinner and they also attend the opera. As time goes by so Julien becomes more careless; on one occasion he pays so little attention to the letter he transcribes that he fails to remove references that imply he is in London! However, persistence pays off when Mathilde discovers the correspondence and Julien's deceit; she faints at his feet. Still smarting from her earlier treatment of him, Julien demands guarantees of her love. Sentiment overtakes them however; that night he attends the opera with Mme de Fervaques, Mathilde is also present, they catch each other's eyes and are overcome by emotion. On their return home the old battle recommences; this time Julien has deduced from the example of Napoleon's strategies the importance of keeping Mathilde in fear of losing him and experience teaches him to disguise his vulnerability.
In any case, the stakes have been raised. Mathilde is pregnant and has no intention of ever leaving him; she writes to her father to beg that they be allowed to marry. Furious, the Marquis summons Julien who incriminates himself, inspired by another hero of his, Molière's Tartuffe (see Act III, scene 6). Julien leaves for the Marquis's property in Villequier, leaving negotiations in the capable hands of Mathilde, who already views herself as his wife. After a lengthy wait, the Marquis gives his consent, bequeathing lands in Languedoc to 'M. le Chevalier Julien Sorel de la Vernaye' as he is henceforth to be known and ensuring him a commission in the cavalry. The Marquis is a remarkably reasonable man; he will not stand in the way of love but needed only time to satisfy himself that Julien had not seduced his daughter for profit. Julien, glorious in his uniform and engaged to the most eligible woman in France has made it. "My novel is finished, and all the glory is mine".
And yet for Julien the true drama is only just beginning. Just as he begins to enjoy the fruits of his new position (for example, he sends the abbé Chélan 500 francs to be distributed amongst the poor) a panic stricken Mathilde informs him that it is all off. The Marquis has received a letter from Mme de Rênal, imposed on her by her confessor, denouncing Julien as a serial seducer, interested only in self-advancement. Julien takes immediate action. He returns to Verrières, buys a pair of pistols, enters the church and fires two shots at Mme de Rênal. He leaves her for dead and is led away by the police.
In jail he is fully aware of the consequences of premeditated murder; he knows he will not escape the guillotine, offers no defence and writes a set of final instructions to Mathilde. Yet the discovery that Mme de Rênal is not in fact dead fills him with joy. He receives visits from a horrified M. Chélan and a bewildered Fouqué, but dreads nothing more than the arrival of his father. The footsteps that seem to herald his arrival actually turn out to belong to Mlle de la Mole, in peasant disguise, who has managed to persuade the abbé Frilair to grant her access. However, Mathilde leaves him cold; he does not believe that she will care for his child. All ambition has faded and he is overwhelmed by deep remorse at having tried to kill Mme de Rênal. Mathilde, who laments that since the revolution it has no longer been possible to rig juries, and a lawyer both try to persuade Julien to construct a defence, yet he refuses. Mme de Rênal also composes a letter to all the jurors, a glowing character reference for the accused, stressing her lack of lust for vengeance. The day of the trial arrives; only the fear that he will die a hated man leads Julien to speak. He confesses his crime - ambition, and the desire to escape wretched poverty. He is condemned to death. Back in his cell, he is visited by a despairing Mathilde but all his thoughts are now with Mme de Rênal. Once again, he refuses to appeal.
To his delighted surprise, he is awoken by Mme de Rênal. They renew their love and she vows to live for his child. In contrast, Mathilde's continual presence serves only to irritate him. He receives the dreaded visit from his father; they reach an agreement over Julien's money, a transaction that terminates their relationship much as it began. Julien is left to his philosophical reflections. Mme de Rênal has now been granted permission to visit him twice a day, much to the disgust of Mathilde; rarely has Julien lived so completely in the present and been so happy. Two events have the potential to disturb his peace: his confessor promises to arrange a pardon if he makes a high-profile conversion to the Jesuits, and Mme de Rênal hatches a plan to appeal to the King in person. He refuses the first offer, and dissuades her from the second. Julien is executed without ceremony. Fouqué guards the decapitated corpse until Mathilde arrives, and reliving the role of her ancestral heroine, kisses the severed head. Julien is buried according to her wishes. Three days later, Mme de Rênal is dead.