Gene Forrester, a man in his 30s, returns after 15 years to the preparatory school attended as a teenager, the Devon School in New Hampshire. He stops at Devon’s main building and looks at a set of marble stairs, marveling at the fact that they seem much smaller than he remembers. At the same time, though, he knows they have stayed exactly the same and that he is the one who has undergone change. Thinking this way, he walks across campus to look at a large tree by the Devon River, once again shocked to find that it is much smaller and less significant than he remembered. This makes him feel glad to have come to Devon, since he now sees that he has grown and gained new perspectives on the hardships he faced as a teenager.
After Gene has this realization, the story shifts to 1942, when he’s about to embark upon his senior year in high school. World War II rages overseas, and the smart and careful Gene and his carefree, athletic roommate Finny are students at Devon’s summer session. One day, Finny, Gene, and some other students hang around a big tree by the river. The school uses this tree as part of its “physical hardening” program, which helps seniors prepare to join the military. Part of this program includes climbing the tree and making the daring plunge from its upper branch into the river, clearing the bank on the way down. Nobody except the seniors have done this, but Finny climbs the tree and jumps. Gene, though terrified, follows. Over time, this act becomes a habit, and Finny proposes that he and Gene form a club, which they name the Super Suicide Society of the Summer Session. One of the rites of passage for joining this secret society is jumping out of the tree. More importantly, Finny and Gene start every meeting by making the plunge themselves, meaning that Gene has to swallow his fear on a very regular basis, since the Super Suicide Society meets each evening.
As summer continues, Gene and Finny grow closer. However, Gene also begins to feel a deep rivalry with Finny, envying his athletic prowess and suspecting his best friend of intentionally trying to distract him from his studies. He thinks this because Finny convinces him one day to ride their bikes to the beach the day before he has an important trigonometry exam. Although Gene would rather study, he goes along with Finny’s idea, and the two boys have a fantastic time and end up sleeping next to one another on the sand. Just before they fall asleep, they look up at the stars and Finny tells Gene that he’s his best friend, and though Gene wants to say the same thing, he can’t bring himself to speak the words. The next day, they barely make it back to campus in time for Gene to take his test, and he fails it miserably. As a result, he privately blames Finny for distracting him, forming a secret belief that Finny wants to sabotage his academic performance because he sees him as a rival.
Despite his theory that Finny wants to interfere with his studies, Gene sees one night that Finny doesn’t actually harbor such feelings. When Finny tries to pull him away from studying in order to go to the tree, he responds angrily. In response, Finny tells him that he should continue to focus on his schoolwork if that’s what he wants—if Finny had the kind of brains that Gene has, he says, he would focus on school, too. This compliment shows Gene that Finny doesn’t see him as a rival, and he bitterly thinks that this is because Finny doesn’t think he’s good enough to be his rival. He then begrudgingly follows Finny to the tree, where Finny suggests that they perform a simultaneous jump. As Finny walks out on the branch, Gene holds onto the trunk and bounces the limb, causing Finny to lose his balance. For a terrible instant, Finny turns around to meet Gene’s eyes, and then he falls to the ground, shattering his leg and his athletic career.
Later, when Finny is in the infirmary, Gene visits him and tries to gauge whether or not he suspects him of malice. At first, Finny insinuates that he does think that Gene may have caused his fall, but he immediately apologizes for jumping to such ridiculous conclusions, admonishing himself for thinking so poorly of his best friend. Overwhelmed, Gene jumps out of his seat and prepares to tell Finny the truth, but Dr. Stanpole—who presides over Devon’s infirmary—swoops in and sends Gene out. Not long after this exchange, Finny is sent home, and the summer session ends.
Gene goes home for summer vacation for just one month before returning to Devon. On the way back, he visits Finny and confesses that he’s responsible for his friend’s injury. Finny refuses to believe him, but Gene goes on at length, passionately insisting that he’s to blame. Eventually, Finny tells him to stop talking, threatening to kill him if he goes on. In this moment, Gene realizes that he’s only hurting Finny even more, so he apologizes and says that he’s not making sense because he’s so tired from his travels. He then returns to Devon and finds that the laxity of the summer session has been replaced by the strict rule of the regular masters.
The senior boys’ consciousness of the war also increases, and soon a boy named Brinker Hadley convinces Gene to enlist after the majority of their class volunteers to shovel snow off some nearby railroad tracks, since so many of the town’s workers have left for the war. Gene agrees to sign up for the military with Brinker the very next day, but all thoughts of actually following through with this plan fade when he returns to his room to find Finny sitting at his desk. The next morning, Brinker enters Gene and Finny’s room and resumes a standing joke that he’s developed, which is that Gene purposefully caused Finny’s fall in order to get their dorm room to himself. When Finny asks Gene what Brinker’s talking about, Gene distracts him by saying that Brinker wants him to enlist. This astounds Finny, whose face betrays disappointment that Gene would ever want to leave him behind. Seeing this, Gene turns on Brinker and tells him that he would never enlist with him—a statement that makes Finny very happy. In a surprising turn of events, though, Finny and Gene’s mild-mannered friend Leper Lepellier enlists in the military, quickly leaving the school behind to join the war effort.
In the aftermath of Finny’s injury, Gene has decided that pursuits like sports feel trivial in light of the war, but Finny argues that war is just a creation of fat old men who want to control the younger generation. Soon Finny convinces Gene to start training for the Olympics—a dream that used to be Finny’s. As Gene’s training intensifies, the two boys regain their closeness and Gene develops a sense of internal peace that he’s never before experienced. One day, Finny proposes that the boys hold a Winter Carnival, where they raucously play in the snow, drink hard cider, and enjoy a prevailing sense of youthful mischief that is, on the whole, quite innocent in comparison to the war. It’s a great success, until a telegram arrives for Gene from Leper. Leper has deserted the military, the telegram explains, and he wants Gene to come meet him at once at his home in Vermont.
When Gene arrives at Leper’s house, he learns that Leper ran away from the military in order to avoid getting discharged for having become mentally unstable. Leper tells Gene that he started hallucinating because of the various stressors of military life, seeing strange images that caused him to scream and cry at random moments. Because he knew that being discharged for mental instability would later make it hard for him to find employment, he ran away. As he talks, Gene becomes increasingly uncomfortable, realizing that the military—for which he and all of his friends are inevitably destined—isn’t as glorious and rewarding as people would like to think. Thinking this way, he tells Leper to stop talking, but Leper refuses, forging on by saying that Gene has always had something “ugly” lurking within him. This, he says, is why Gene caused Finny to fall from the tree. Hearing this, Gene kicks Leper out of his chair, and though the two boys make up, he leaves shortly thereafter, deeply troubled by what Leper has said.
Back at Devon, Brinker begins to question why Gene hasn’t enlisted and suspects it has something to do with Finny’s fall, suggesting that Gene feels guilty for what happened to his best friend and therefore doesn’t want to leave him behind by enlisting. What’s more, the story of what happened to Leper leads both Finny and Gene to admit that the war is indeed real, since only something serious could cause somebody like Leper to have such an adverse reaction. Around this time, Finny spots Leper hiding behind a bush before running into a teacher’s room. That night, Brinker and a few other students round up Gene and Finny and hold a mock trial to investigate Finny’s fall. Although Gene resists this, they proceed, asking Finny what he remembers about that evening. Eventually, they bring in Leper as a witness, and the information he gives them about what happened in the tree makes it clear to everyone—including Finny—that Gene purposefully caused the fall. In response, Finny stands up and rushes out of the room, making his way down the hall before crashing down the nearby set of marble stairs and re-breaking his leg.
Gene tries to visit Finny in the infirmary that night, but Finny is furious at him, so he spends the night wandering campus and feeling as if his surroundings are strange and foreign to him. The next morning he visits Finny once more, and together the two boys agree that Gene’s actions at the tree were not purposeful. Finny also admits that he has spent the entire year writing to various military organizations, desperately wanting to find one that would allow him to contribute to the war effort. Because they all denied his requests, though, he decided to ignore the war altogether.
Finny dies that afternoon after a small amount of his bone marrow enters his bloodstream. Dr. Stanpole tells Gene that he will most likely have to get used to hearing this kind of bad news, since he and his friends are headed for the war. Despite this devastating turn of events, though, Gene doesn’t cry, nor does he shed a tear at the funeral, since he feels that Finny is now a part of him and he believes that one does not cry at one’s own funeral.
In the aftermath of Finny’s death, Gene and his classmates graduate and enlist in the safest branches of the military. Gene never sees active duty, but feels that he fought his own war at Devon and that he understands the hatred all men harbor in their heart—all men, that is, except Finny.