At the start of the story, Jake Barnes, an American journalist and war veteran, is living a somewhat bohemian lifestyle in Paris. He is in love with Lady Brett Ashley, a young English war widow, but their relationship is complicated due to Jake's war injury that has left him sexually incapacitated. Brett, for convenience, has become engaged to Michael Campbell, an Englishman. Robert Cohn, a young American writer and former boxing champion from Princeton University, is also drawn to Brett. The expatriates travel to Pamplona for the Fiesta de San Fermin, where they meet the young matador Pedro Romero, who performs "without falsity," upholding the pure standards of bullfighting. Sexual intrigue, mainly centered around Brett, triggers Jake's reflection on his generation's moral standing.

Hemingway selects two contrasting epigraphs for The Sun Also Rises, establishing a clear and straightforward theme through their juxtaposition. Gertrude Stein, a writer and mentor to many young artists in 1920s Paris, remarked of the American expatriates: "You are all a lost generation." Stein's comment implies the fleeting nature of humankind; Hemingway interpreted it to mean that his generation no longer had access to the ideals and structured order of pre-World War I civilization.

She gazed into my eyes in that unique way she had, making you question whether she truly saw through her own eyes.
Hemingway's second epigraph is from the Old Testament Book of Ecclesiastes: "One generation passeth away, and another generation cometh; but the earth abideth forever...The sun also ariseth, and the sun goeth down." This prophecy implies a cosmic order: in God's scheme, there is no "lost generation," and the self-centered, fragile human ego seems insignificant compared to the cycles of the sun and the passage of time. Until Jake, Brett, and the others realize they are lost only because they lack the moral strength to subordinate personal desires to universal truths, their lives will remain meaningless. Hemingway stated that he did not intend for The Sun Also Rises to be "a hollow or bitter satire, but a damn tragedy with the earth abiding forever as the hero."

Amidst the tragedy, Hemingway intersperses scenes of happiness and celebration: the gaiety of Paris nightlife; the splendor of Pamplona during its festival, with bustling crowds and noble matadors; and the tranquility of the Basque countryside where Jake and Bill Gorton hike and fish. Behind the nightlife lies alcoholism, behind the bullfights tragic realities, and behind the fishing an unconscious quest for simplicity by a generation. The fishing scenes, far from being mere pastoral interludes in an otherwise frantic novel, reinforce Hemingway's primary theme; Jake realizes he is missing something crucial to his happiness and embarks on a quest to rediscover his generation's lost values.

Jake finds his greatest happiness when he's fishing, watching bullfights, or riding a rundown bus while drinking wine and practicing his Spanish with the locals. The owner of the Hotel Montoya in Pamplona understands Jake more deeply than any of his "friends" due to their shared passion as aficionados. Ultimately, Jake sacrifices his most valued possession—his status as an aficionado—by introducing young Romero to Brett's seduction. Thus, Jake represents a generation that has matured only to find personal peace even more elusive than global peace.

A central theme in much of Hemingway's fiction is the concept of "grace under pressure," a code through which individuals can bring honor to themselves. In essence, this code dictates that, regardless of the circumstances, a person must not falter. The grace in question encompasses physical, aesthetic, moral, and spiritual aspects; the challenges to be endured include complex moral decisions, the chaos of violence, and, most importantly, the ever-present reality of death that haunts every human being. In The Sun Also Rises, the primary representation of death is the bullfight, where the matadors exhibit exemplary behavior rooted in courage, honor, and passion. During the ritual of the bullfight, both the bull and the matador confront uncertainty and face death with equal dignity and bravery.

Hemingway once claimed that The Sun Also Rises was the most moral book he had ever written, describing it as a "tract against promiscuity." Although it initially appears that Brett is willing to sleep with anyone, she ultimately realizes that she must not corrupt Romero. By acknowledging the importance of maintaining standards, Brett reflects the novel's central theme—the necessity of discovering or rediscovering values that define a morally fulfilling life.