On September 15, 1840, eighteen-year-old Frederick Moreau boards a steamboat departing Paris for Montereau, returning home to the provincial town of Nogent-sur-Seine after visiting a wealthy uncle in Havre. On deck, he meets Jacques Arnoux, the jovial proprietor of
L'Art Industriel, a combination art journal and picture shop, and then catches sight of Arnoux's wife, a dark-haired woman of striking beauty seated alone on a bench. Frederick is immediately captivated. He rescues her shawl when it nearly falls into the water, and their eyes meet briefly. At the landing, the Arnouxs depart, and Frederick watches Madame Arnoux until the boat disappears. His mother, Madame Moreau, a proud woman of noble lineage whose late husband left a much encumbered estate, has ambitious plans for Frederick's legal career.
That night, Frederick's closest friend, Charles Deslauriers, visits. The impoverished, fiercely ambitious son of an ex-army captain, Deslauriers has been Frederick's confidant since their school days. He cannot yet afford to move to Paris and urges Frederick to cultivate both Arnoux and the wealthy banker M. Dambreuse. Two months later in Paris, Frederick attends law lectures halfheartedly and sinks into idleness, obsessively watching the Arnoux apartment from afar. Through a student disturbance, he meets Hussonnet, a witty journalist, and Dussardier, a big-hearted shopman. Hussonnet brings Frederick into Arnoux's artistic circle, including Pellerin, a frustrated painter, and Mademoiselle Vatnaz, a sharp-tongued woman involved in Arnoux's tangled business affairs.
When Deslauriers arrives in Paris with his savings, Frederick welcomes him but abandons him that same evening for a dinner invitation at the Arnouxs'. Madame Arnoux sings an Italian aria that overwhelms Frederick, and he begins attending weekly dinners, his emotional life revolving around stolen moments in her presence. Deslauriers organizes Saturday gatherings at their shared apartment, where friends debate politics and art: the rigid republican Sénécal, the cautious law student Martinon, and the young aristocrat Cisy among them. Frederick's mother then reveals their dire finances, roughly ten thousand francs in yearly income. He takes a miserable clerkship in Nogent and endures years of provincial tedium, befriending the young Louise Roque, Père Roque's spirited daughter, who develops a passionate attachment to him. On December 12, 1845, Frederick learns his uncle has died intestate, leaving him a substantial fortune, and returns to Paris immediately.
Frederick undertakes a comic search for the Arnouxs, who have moved. He finds them at a new address; Arnoux now deals in faience, a type of decorative pottery. Frederick furnishes an elegant apartment and begins circulating in society, calling on the Dambreuses and visiting Rosanette Bron, a vivacious courtesan known as "the Maréchale," a nickname meaning the Marshal's wife. He attends a costume ball at Rosanette's home and hosts a lavish housewarming for his friends. Learning of Arnoux's failing finances, he positions himself as Madame Arnoux's confidant and protector.
When Arnoux desperately needs money, Frederick gives him fifteen thousand francs from a check intended for Deslauriers's journal project, lying that he lost the money gambling. A bitter rift opens between the friends. Deslauriers visits Madame Arnoux under a legal pretext and attempts to seduce her; she rebuffs him coldly. To wound her, he claims Frederick is about to marry Louise. Alone afterward, Madame Arnoux realizes with sudden clarity that she loves Frederick.
Frederick accompanies Rosanette to the races, where Madame Arnoux glimpses him with his mistress and the Dambreuses observe the scene. At a dinner party, Frederick defends Arnoux's honor when Cisy slanders Madame Arnoux, hurling a plate at his face. A duel is arranged, but Cisy faints at the dueling ground before a blow is struck.
Frederick now moves among three distinct worlds, entangled with three women: Madame Arnoux, with whom he shares suppressed passion; Rosanette, whose sensuality draws him in; and Madame Dambreuse, whose aristocratic elegance appeals to his ambition. His speculations on the stock market first succeed but then fail, sharply reducing his income. After months of deepening intimacy, Frederick and Madame Arnoux agree to a rendezvous on the afternoon of February 22, 1848. He rents and lavishly furnishes a room. On the appointed day, as revolution erupts across Paris, she does not come: Her son Eugène has fallen gravely ill, and she stays at his bedside, interpreting the crisis as divine punishment for her intended adultery. She vows to sacrifice her passion forever. Frederick, agonized, stumbles through the revolutionary chaos to Rosanette's door and takes her to the room prepared for Madame Arnoux, consummating their affair amid violets and lace meant for another woman.
The February Revolution overthrows King Louis-Philippe. Frederick witnesses the storming of the Tuileries palace and the declaration of the Republic. M. Dambreuse reinvents himself as a Republican. During the bloody June Days, Frederick and Rosanette flee to Fontainebleau for an idyllic interlude while civil war rages in the capital. Dussardier lies wounded and confesses doubts about whether the Republic has betrayed the people.
Frederick encounters Madame Arnoux alone in the Arnoux warehouse and declares his love. She is moved, but the moment is interrupted. In a later meeting in her drawing room, they reconcile and embrace in a long kiss, only for Rosanette to appear and address Frederick with familiar intimacy, devastating Madame Arnoux. Rosanette then reveals she is pregnant, preventing a rupture. Frederick settles into domestic life with her while beginning a secret affair with Madame Dambreuse. M. Dambreuse dies, and his wife discovers the will leaves everything to Cécile, Dambreuse's natural daughter, reducing her own inheritance sharply. Despite his disappointment, Frederick accepts Madame Dambreuse's proposal of marriage.
Rosanette gives birth to a sickly boy who soon dies. Frederick obtains money from Madame Dambreuse to save Arnoux from arrest but arrives too late: The family has fled Paris. Madame Arnoux's furniture is put up for auction, the result of proceedings engineered by Sénécal and facilitated by Deslauriers. Frederick watches in agony as her belongings are sold piece by piece. Madame Dambreuse deliberately bids on a cherished silver chest and wins it. Revolted, Frederick breaks with her on the spot and later leaves Rosanette for good.
On December 2, 1851, the day of Louis-Napoléon's coup d'état, Frederick travels to Nogent intending to reconcile with Louise, only to witness her emerging from the church as the bride of Deslauriers. In Paris, on the boulevards amid cavalry charges, he sees Dussardier standing defiantly, shouting "Long live the Republic!" A police officer strikes Dussardier down. Frederick recognizes the officer as Sénécal, the former rigid republican turned agent of authoritarian power.
Frederick travels abroad for years, aimless and melancholy. Toward the end of March 1867, Madame Arnoux visits him unexpectedly, now white-haired and living in modest retirement in Brittany with her husband, who is an invalid. They reminisce about their shared past, and she confesses she had always loved him. She seems to offer herself, but Frederick, feeling the weight of his idealized love, draws back. She cuts a lock of her hair for him and departs forever.
In a final scene, Frederick and Deslauriers, reconciled after Deslauriers's marriage to Louise collapsed when she eloped with a singer, sit by the fire and review their failed lives. Deslauriers tells Frederick that Arnoux died a year ago. They recall their adolescent visit to a brothel in Nogent, when Frederick was so overcome with embarrassment that he fled without entering. Both agree: "That was the best time we ever had."