Jake Barnes, the narrator, describes his friend, Robert Cohn. Cohn, like Jake, is an American expatriate living in Paris, although unlike Jake he did not fight in World War I. He's a Jewish writer who has recently published a novel and was a middleweight boxing champion in college at Princeton. Cohn lives with a woman named Frances Clyne, who was originally using him for his money but now that she's older wants to make him marry her. After reading a book that romanticizes travel Cohn has come to the conclusion that he's wasting his life, and one day he visits Jake, who is a journalist, at his office to ask him to take a trip with him to South America. Jake refuses, on the grounds that the only people who don't waste their lives are bullfighters.
That night, while out with Cohn and others, Jake runs into Lady Brett Ashley. Brett is an independent, tomboy-ish, soon-to-be-divorced wife of an English lord who as a volunteer during the war helped to treat Jake for a war wound he received. While she was treating him, they fell in love. Brett confesses to Jake that she is miserable and still loves him, just as he loves her. But though they never say it explicitly, the conversation implies that Jake's injury made him impotent, and that Brett is unwilling to give up sex, so they can't be together. Still, they make plans to see each other the next afternoon.
Jake has lunch with Cohn the next morning. Cohn is smitten by Brett, and is upset when Jake describes her in less-then-positive terms (and also when he learns that she's soon to marry a Scottish war veteran named Mike Campbell). Brett stands Jake up for their afternoon plans. But in the middle of the night she appears at his apartment along with Count Mippipopolous, a rich Greek man who really knows how to enjoy life. In a moment when they are alone, Brett tells Jake that it's too hard for them to be near each other and that she's leaving the next day to go to San Sebastian, a beach town in Spain. Cohn also leaves Paris around this time to spend time out in the country.
A few weeks later, a writer and army-friend of Jake's named Bill Gorton arrives in Paris. They plan to go fishing in Spain and then to go to the fiesta and bullfights in Pamplona, and to join up with Cohn along the way. That afternoon, Jake runs into Brett, who has returned from San Sebastian and is with her fiancé Mike. Mike and Brett also want to come to Pamplona. Brett privately asks Jake if Cohn is also coming, revealing that she was actually with Cohn in San Sebastian.
Bill and Jake meet Cohn in Bayonne, France, and then all three travel to Pamplona. Brett, however, falls ill while traveling to Pamplona with Mike. Cohn decides to stay in Pamplona to wait for her while Jake and Bill head out into rural Spain to fish. For five blissful days Jake and Bill fish, play cards, drink, and remember their days and friends from the army. But on the fifth day they learn that Brett and Mike will be arriving in Pamplona that night, and they immediately head back.
In Pamplona, they stay at a hotel owned by Montoya, a man who loves bullfighting and appreciates Jake's own love of the sport. Jake, Bill, Cohn, Mike, and Brett all meet up. They go to watch the unloading of the bulls, and see a bull kill a steer. Afterward, Mike compares Cohn to the steer because Cohn won't stop following Brett around.
The fiesta begins, and Pamplona is filled with drinking and dancing. During the bullfights on the first day, a nineteen-year-old bullfighter named Pedro Romero especially stands out. Brett is mesmerized by the violence of the fight (while Cohn is made ill by it). Brett is also particularly taken with Romero. Brett eventually gets Jake to introduce her to Romero, much to Montoya's dismay because he thinks she will corrupt the boy. Mike again verbally attacks Cohn, and they almost fight before Jake pulls them apart. Later that night, Brett asks Jake to help her find Romero. He does, and she and Romero go off together.
Later that night, while Jake is out with the drunken Mike and Bill, Cohn arrives and demands to know where Brett is. After Jake refuses and insults fly, Cohn knocks down Mike and knocks Jake out cold. When Jake comes to and returns to the hotel, he finds Cohn weeping in his room. Cohn begs Jake's forgiveness. After some resistance, Jake gives it. Cohn says he is leaving Pamplona.
The next morning, a man is killed by a bull outside the bullfighting stadium. Soon after, Jake learns from Bill and Mike that the night before Cohn also beat up Romero, but Romero wouldn't back down. Cohn gave in, and asked Romero to forgive him, but Romero just punched him. At the bullfight that afternoon, a bullfighter who had come out of retirement named Belmonte fails to live up to his reputation and is jeered by the crowd. But Romero fights magnificently, and the crowd adores him. Later that night, Jake learns from Mike that Romero and Brett have left Pamplona together.
The fiesta ends the next day. Jake, Mike, and Bill leave Pamplona together, then go their separate ways. Jake decides to lay low in San Sebastian rather than return to Paris. But he soon gets a telegram from Brett saying she needs his help in Madrid. He goes immediately, and learns that Brett has left Romero because she feared corrupting him but also because he wanted her to act like a more traditional woman. As they ride in a taxi through Madrid, Brett sadly comments that she and Jake could have had such a good time together. Jake says, "Yes, isn't it pretty to think so?"