M. de Rênal, ultra mayor of the small provincial town of Verrières, hires Julien Sorel, a young peasant who aspires to the priesthood, as tutor for his children. The hiring of Julien is calculated to enhance Rênal's prestige among the wealthy liberals. Julien, ambitious and amoral, had hoped to pursue a military career but has decided to enter the priesthood as the most likely means to success. He chooses hypocrisy as his weapon in his encounter with society. He sees his position as tutor as the first step in his ascension, which will culminate, he hopes, in Parisian aristocracy.
Mme. de Rênal innocently falls in love with Julien after he has lived in the Rênal country home for some time. When Julien discovers that he is loved, he decides that he will seduce Mme. de Rênal as an expression of the scorn he feels for her husband. His plan of seduction would have failed miserably, so awkwardly does he execute it, were Mme. de Rênal not hopelessly in love with him. Succumbing to Julien's natural charm, which he displays in unguarded moments, Mme. de Rênal becomes, in fact, Julien's mistress. She educates him socially and in the local political intrigues. She succeeds in having Julien awarded a much coveted place in the guard of honor on the occasion of a visit by Charles X.
Their love affair is idyllic until one of the Rênals' sons falls gravely ill, which Mme. de Rênal interprets as divine punishment for her adultery. Soon M. de Rênal receives an anonymous letter accusing Julien of having seduced his wife. Mme. de Rênal succeeds in duping her husband into believing that the accusation is false. She convinces him that the letter comes from Valenod, Rênal's rival and assistant, who has attempted in the past to court Mme. de Rênal. Her husband believes her because he is comfortably established and is horrified at the thought of a scandal. In order to quiet the rumors, Julien moves into the Rênals' townhouse in Verrières. Because of his brilliant reputation as a tutor, he is invited to dinner by Valenod, who would hope to hire Julien as the tutor for his own children.
A servant girl from the Rênal household, also in love with Julien but spurned by him, denounces the lovers to the former village priest, Chélan, who insists that Julien leave Verrières to enter the seminary in Besançon. Through Chélan's influence with Pirard, rector of the seminary, Julien is awarded a scholarship. Julien's affair with Mme. de Rênal is temporarily ended, but he visits her room for a final rendezvous.
Julien's first attempts to succeed as a student meet with failure because he excels as a scholar, and the Church's reactionary influence that prevails in the seminary requires of its future priests docility and intellectual conformity in mediocrity. Julien's superiority, however, is appreciated by Rector Pirard, who makes Julien his protégé. One day as Julien is assisting in the decoration of the Besançon cathedral, he encounters Mme. de Rênal, who promptly faints at the sight of him.
Pirard obtains a position for Julien as secretary to a powerful aristocrat in Paris, the Marquis de La Mole, to whom Pirard has been of invaluable assistance in a lawsuit. Pirard also leaves Besançon for a comfortable parish in Paris.
Before going to Paris, Julien pays a last visit to Mme. de Rênal, presenting himself at her window late at night. At first rebuffed by his mistress' virtue, Julien artfully destroys her resistance by announcing that his departure for Paris is imminent and that they will never see each other again. Mme. de Rênal acquiesces and Julien remains hidden to spend the following day with her.
Book II finds Julien in Paris as secretary to the Marquis de La Mole. Soon Julien makes his services indispensable to his employer, although his provincial manners and inexperience in high society cause him constant embarrassment. The marquis' proud daughter, Mathilde, takes an interest in Julien when she overhears the latter denouncing the sterility of the Mole's salon. Mathilde is bored with the convention and barrenness of the aristocracy of which she is a part. She is in need of diversion, and Julien will provide it for her. The marquis finds Julien's intelligence and wit very refreshing, and ultimately Julien becomes almost a son to the marquis. The latter sends Julien to London on a diplomatic mission in order that he may gain experience and as a pretext to have Julien awarded a decoration.
At the behest of Mathilde, Julien attends a ball, where he makes the acquaintance of a liberal aristocrat condemned to die. Mathilde is the most sought-after beauty of the season, but Julien hardly notices her, so inspired is he by the hero he has met. Mathilde, on the other hand, sees in Julien a reincarnation of her illustrious ancestor, Boniface de La Mole, a queen's lover who was beheaded. Mathilde falls in love with Julien.
Julien is unable to decide if he is loved or if Mathilde and her brother and their friends are trying to make of him a dupe. Julien's attempt to leave Paris on a business trip for the marquis moves Mathilde to a declaration of love. Julien, still distrustful, takes precautions to safeguard his reputation, sending Mathilde's avowal to his friend, Fouqué. Alleging another business trip, Julien receives an invitation from Mathilde to visit her in her room late at night. Still convinced that he is being tricked, Julien nonetheless appears at the appointed hour, and after much mutual embarrassment, Mathilde becomes his mistress.
Mathilde now fears that she has given herself a master, and she repents of having compromised herself. Julien discovers that he is desperately in love with Mathilde, but her ardor has cooled. Unfortunately for Julien, Mathilde is only capable of loving him when she thinks that she is not loved by him. When in a moment of anger Julien one day appears to threaten her life, she is in love again. Their second rendezvous occurs, but Mathilde again repents immediately after.
Julien, tormented by passion, is called upon by the marquis to serve as secretary at a secret meeting of reactionary aristocrats and to deliver a secret message to London. Successfully fulfilling his mission, Julien then goes to Strasbourg, where he meets a former acquaintance from London, who advises him how to reawaken Mathilde's love by jealousy. Julien returns to Paris to execute his plan, choosing a prude to court by means of love letters furnished to him by his friend.
Mathilde responds to the stratagem, but Julien realizes that to keep her love alive he must love her at a distance. Mathilde is pregnant, and after the marquis' rage has subsided at the announcement of this news, the latter finally agrees to obtain an army commission for Julien and to encourage his career. Julien occupies his new post in Strasbourg but receives word from Mathilde to return to Paris, that all is lost. In checking on Julien's past, the marquis has learned from Mme. de Rênal, in a letter dictated by her confessor, that Julien is an opportunist who succeeds by seducing women.
Learning this, Julien hurries to Verrières, arms himself, and shoots Mme. de Rênal at church. Imprisoned and awaiting trial for attempted murder, Julien is visited by Mathilde, who attempts to negotiate his acquittal with the Jesuits. Julien is resigned to die and in the solitude of his prison cell discovers that he is still in love with Mme. de Rênal, whom he had only wounded, and that his love for Mathilde has disappeared.
During the trial, in spite of his resolution not to speak in his own defense, Julien informs the court that he is not being tried for attempted murder but for having attempted to rise above his social class. The jury finds Julien guilty and he is sentenced to be guillotined.
During his last days in prison, Julien finds peace and happiness in his reflections and through the reunion with Mme. de Rênal, who visits him daily. Julien faces death courageously, and after the execution, Mathilde, in a re-enactment of a scene from the Mole family history, furtively steals Julien's severed head and lovingly buries it with her own hands. Mme. de Rênal follows Julien in death.