On the surface, the novel is a love story between the protagonist Jake Barnes—a man whose war wound has made him unable to have sex—and the promiscuous divorcée Lady Brett Ashley. Jake is an expatriate American journalist living in Paris, while Brett is a twice-divorced Englishwoman with bobbed hair and numerous love affairs, and embodies the new sexual freedom of the 1920s. Brett's affair with Jake's Princeton friend Robert Cohn (whom the characters often refer to by his last name) causes Jake to be upset and break off his friendship with Robert; her seduction of the 19-year-old matador Romero causes Jake to lose his good reputation among the Spaniards in Pamplona.
Book One is set in the café society of young American expatriates in Paris. In the opening scenes, Jake plays tennis with Robert, picks up a prostitute named 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 at Bayonne for a fishing trip in the hills northeast of Pamplona. Instead of fishing, Robert stays in Pamplona to wait for the overdue Brett and Mike. Robert 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. Robert is resented by the others, who taunt him with antisemitic remarks. During the fiesta, the characters drink, eat, watch the running of the bulls, attend bullfights, and bicker. 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, Mike, Robert, and Romero each want Brett. Robert, who had been a champion boxer in college, has a fistfight with Jake and Mike, and another with 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 on the northern coast of 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 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 and thinking of what might have been.
