On the surface the novel is a love story between the protagonist American Jake Barnes—a man whose war wound has made him impotent—and the promiscuous divorcée Lady Brett Ashley. Brett's affair with Robert Cohn causes Jake to be upset and break off his friendship with Cohn; her seduction of the 19-year-old matador Romero causes Jake to lose his good reputation among the Spaniards in Pamplona.
Barnes is an expatriate American journalist living in Paris. He is in love with Lady Brett Ashley, a twice-divorced Englishwoman. Brett, with her bobbed hair and numerous love affairs, embodies the new sexual freedom of the 1920s.
Book One is set in the café society of young American expatriates in Paris. In the opening scenes, Jake plays tennis with his college friend Robert Cohn, picks up a prostitute (Georgette), and runs into Brett and Count Mippipopolous in a nightclub. Later, Brett tells Jake she loves him, but they both know that they have no chance at a stable relationship.
In Book Two, Jake is joined by Bill Gorton, recently arrived from New York, and Brett's fiancé Mike Campbell, who arrives from Scotland. Jake and Bill travel south and meet Robert Cohn at Bayonne for a fishing trip in the hills northeast of Pamplona. Instead of fishing, Cohn stays in Pamplona to wait for the overdue Brett and Mike. Cohn had an affair with Brett a few weeks earlier and still feels possessive of her despite her engagement to Mike. After Jake and Bill enjoy five days of fishing the streams near Burguete, they rejoin the group in Pamplona.
All begin to drink heavily. Cohn is resented by the others, who taunt him with anti-semitic remarks. During the fiesta the characters drink, eat, watch the running of the bulls, attend bullfights, and bicker with each other. Jake introduces Brett to the 19-year-old matador Romero at the Hotel Montoya; she is smitten with him and seduces him. The jealous tension among the men builds—Jake, Campbell, Cohn, and Romero each want Brett. Cohn, who had been a champion boxer in college, has fistfights with Jake, Mike, and Romero, whom he beats up. Despite his injuries, Romero continues to perform brilliantly in the bullring.
Book Three shows the characters in the aftermath of the fiesta. Sober again, they leave Pamplona; Bill returns to Paris, Mike stays in Bayonne, and Jake goes to San Sebastián in northeastern Spain. As Jake is about to return to Paris, he receives a telegram from Brett asking for help; she had gone to Madrid with Romero. He finds her there in a cheap hotel, without money, and without Romero. She announces she has decided to go back to Mike. The novel ends with Jake and Brett in a taxi speaking of the things that might have been.