Chapter I of The Sun Also Rises introduces us to the novelist Robert Cohn, a graduate of Princeton University who married a wealthy woman and founded a literary journal soon after college. When Cohn's wife left him, he became involved with a woman named Frances Clyne, and they traveled together to Paris, where they are living at the start of the novel's action. It is the mid-1920's.
Cohn visits the story's narrator and main character, Jake Barnes, in the Paris offices of the newspaper for which Jake works. Later, Jake picks up a prostitute named Georgette, and the two of them join a group including Cohn, Frances, and some others. The group goes dancing at a nightclub, where a woman named Brett (also known as Lady Ashley, because she is, by marriage, a titled British aristocrat) appears. Cohn is attracted to Brett, but she leaves the club with Jake.
Jake tries to kiss Brett, but she withdraws, telling him that although she loves him, she "can't stand it." (Apparently, Jake has been castrated in combat during the Great War and cannot consummate his love for Brett.) They rejoin their friends and are joined in turn by a Greek Count named Mippipopolous before Jake returns to his apartment, where he lies in bed, drunk and miserable. The next day, Cohn speculates that he may be in love with Brett, and Frances tells Jake that she believes Cohn plans to break up with her.
When Brett and the Count visit Jake's apartment, Jake tells Brett he loves her and asks if they can live together. She replies that doing so is impossible because she would be tempted to cheat on him. She also tells him that she is about to travel to San Sebastian, a coastal town in the Basque region of Spain. Later, Brett admits to Jake that she feels miserable, apparently due to her unfulfilled love for him.
Jake receives a postcard from Brett in San Sebastian, as well as a note from Cohn saying that he's leaving the country for a while; it is rumored that Frances has gone to England. Jake's friend Bill Gorton visits Paris, severely intoxicated. They are joined by Brett, back from San Sebastian, and Mike Campbell, her fiancé. Mike, too, is falling-down drunk.
Jake writes to Robert Cohn in Spain to say that he and Bill will meet Cohn at Bayonne (near the French-Spanish border) to go fishing together near the Spanish village of Burguete. Mike invites himself and Brett along, and they arrange to rendezvous in the nearby town of Pamplona. Then Brett reveals to Jake that Cohn was with her in San Sebastian. Jake and Bill depart Paris via rail and arrive in Bayonne in the evening. The next morning, Jake, Bill, and Cohn travel to Pamplona; however, Brett and Mike are not on the train they were scheduled to take. The following day, Jake and Bill go fishing as planned. Cohn has announced his decision to remain in Pamplona. For five days, Jake and Bill hear nothing from Cohn, Brett, or Mike. While fishing, they befriend an Englishman named Harris.
After receiving telegrams from both Mike and Cohn, however, Jake and Bill return to Pamplona. There they meet up with Brett, Mike, and Cohn before walking to the corrals outside of town to see the unloading of the bulls for the coming bullfights. At a café afterward, Mike browbeats Cohn for tagging after Brett. Apparently, Cohn returned to San Sebastian while Jake and Bill were fishing in Burguete.
Pamplona's yearly fiesta of San Fermin, which will last for seven days, begins. Musicians and dancers fill the streets and shops — including the wine store, where Brett is placed on a cask so the Basque peasants can dance around her as if she were a pagan idol. Jake sleeps while his friends stay out all night and then attend the running of the bulls from the corrals to the bullring, through the streets of town. Jake meets the 19-year-old matador Pedro Romero, and the next day, after Romero performs admirably in the ring, Brett cannot help talking about her attraction to him.
The hotelier Montoya visits Jake to express his concern that mixing with rich tourists will corrupt Romero. Later, at dinner, Brett invites the bullfighter to her table. Montoya looks on with disapproval. Once again, Mike picks on Cohn. Brett too lashes out at him, then tells Jake that she feels guilty for having slept with Cohn while engaged to Mike. She asks Jake if he loves her, and when Jake says that he does, Brett says she is in love with Romero. So Jake helps Brett find the matador in a café, where they flirt openly. Jake leaves; when he returns, the two are gone. Later, Cohn calls Jake a pimp, and in the ensuing fistfight, Jake is beaten up. The next day, Mike reports that Cohn found Brett in Romero's room and beat the bullfighter up, too, after which Cohn cried. Mike admits that he is upset by his fiancée's promiscuity.
On the last day of the fiesta, Cohn has left town, presumably to return to Frances. Jake and Brett pray at the Pamplona cathedral before she visits Romero. Then Jake, Brett, and Bill attend the bullfight, in which Romero, beloved of the crowd, performs spectacularly. Brett leaves town also, in the company of the matador.
The rest of the group splits up. Jake travels to San Sebastian, where he relaxes alone in cafés and on the beach. Soon, however, a telegram from Brett arrives, begging him to join her in Madrid. Jake finds Brett in her hotel room there, devastated by the end of her affair with Romero. Brett reveals that it was she who ended the relationship, and that she intends to return to Mike.