Frédéric Moreau renews his acquaintance with a childhood friend, Deslauriers, who advises him to meet with Dambreuse, a rich Parisian banker. Frédéric leaves for Paris, armed with a letter of recommendation from his neighbour M. Roque, who works for Dambreuse. Despite this, his introduction to Dambreuse is not very successful. In Paris, Frédéric stumbles across a shop belonging to M. Arnoux, whose wife he developed a fascination for when he met her briefly at the start of the novel. However, he does not act on his discovery, and lives idly in Paris for some months. A little more than a year after the start of the story, Frédéric is at a student protest and meets Hussonet, who works at M. Arnoux's shop. Frédéric becomes part of a group of friends who meet at the shop. Eventually, he is invited to dinner with M. and Mme Arnoux. At the same time, his old friend Deslauriers comes to Paris. Frédéric becomes obsessed with Mme. Arnoux. Deslauriers tries to distract him by taking him to a cabaret, where they encounter M. Arnoux and his mistress Mlle Vatnaz. Later, Frédéric is persuaded to return home to his mother, who is having financial difficulties. At home, he meets Louise, the daughter of his neighbour M. Roque. His financial worries are eased by the chance death of an uncle, and he leaves again for Paris.
Returning to Paris, Frédéric finds that M. and Mme Arnoux no longer live where they used to. He searches the city, eventually meeting Regimbart, one of his group of friends. He learns that Arnoux has financial problems and is now a pottery merchant. Arnoux introduces Frédéric to another of his mistresses, Rosanette. Frédéric likes Rosanette, and has Pellerin paint him a portrait of her. Mme Arnoux learns of her husband's infidelity. Frédéric has promised money to Deslauriers, but has to lend it to Arnoux instead, who is unable to repay him. Deslauriers and Frédéric fall out. In an attempt to resolve the financial situation, Frédéric returns to Dambreuse, who this time offers him a position. However, Frédéric fails to keep his appointment, instead visiting Mme Arnoux at the pottery factory. She is unresponsive to his advances, and on his return to Paris he instead pursues Rosanette. His difficulties mount and eventually he meets again with Deslauriers, who advises him to return home. At home, Frédéric falls in love with and becomes engaged to Louise, his neighbour's daughter. Deslauriers conveys this news to Mme Arnoux, who is upset. Frédéric says he has business to complete in Paris. While there, he meets Mme Arnoux, and they admit their love for one another. They arrange to meet in private, but Mme Arnoux's son falls seriously ill. Upset and unaware of the reason for Mme Arnoux's absence, Frédéric sleeps with Rosanette instead.
In the midst of the revolution, Frédéric's political writings win him the renewed respect of his friends and of M. Dambreuse. Frédéric, living with Rosanette, becomes jealous of her continued friendship with M. Arnoux, and persuades her to leave with him for the countryside. On his return, Frédéric dines at the Dambreuse's house with Louise and her father, who have come to Paris to find him. Louise learns of Frédéric's relationship with Rosanette. Frédéric meets with Mme Arnoux, who explains why she missed their arranged meeting. During this encounter, Rosanette appears and reveals she is pregnant. Frédéric decides to seduce Mme Dambreuse, in order to gain social standing. He is successful, and soon afterwards M. Dambreuse dies. Rosanette's newborn child becomes severely ill and lives only a short time. Meanwhile, M. Arnoux has finally been overtaken by his financial difficulties and is preparing to flee the country. Unable to face the loss of Mme Arnoux, Frédéric asks for money from Mme Dambreuse, but is too late to stop M. and Mme Arnoux leaving. Mme Dambreuse meanwhile discovers his motive for borrowing the money. Frédéric returns to his childhood home, hoping to find Louise there, but discovers that she has given up on him and married Deslauriers instead. Frédéric goes back to Paris. Many years later, he briefly meets Mme Arnoux again, swearing his eternal love. After another interlude, he encounters Deslauriers, and the novel ends the way it began, with the pair swapping stories of the past.