Gene Forrester is a quiet, intellectual student at the Devon School in New Hampshire. During the summer session of 1942, he becomes close friends with his daredevil roommate Finny, whose innate charisma consistently allows him to get away with mischief. Finny prods Gene into making a dangerous jump out of a tree into a river, and the two start a secret society based on this ritual. Gene gradually begins to envy Finny’s astonishing athletic abilities, manifested in Finny’s breaking a school swimming record on his first try. He thinks that Finny, in turn, envies his superior academic achievements, and he suspects that his friend has been taking steps to distract him from his studies. Gene’s suspicions transform into resentful hatred, but he nevertheless carefully maintains an appearance of friendship.
Gene realizes that he has been grievously mistaken about the existence of any rivalry between them when, one day, Finny expresses a sincere desire to see Gene succeed. While still in a state of shock from the force of his realization, he accompanies Finny to the tree for their jumping ritual. When Finny reaches the edge of the branch, Gene’s knees bend, shaking the branch and causing Finny to fall to the bank and shatter his leg. The tragedy is generally considered an accident, and no one thinks to blame Gene—especially not Finny. But when the doctor tells Gene that Finny’s athletic days are over, Gene feels a piercing sense of guilt. He goes to see Finny and begins to admit his part in Finny’s fall, but the doctor interrupts him, and Finny is sent home before Gene gets another chance to confess.
The summer session ends, and Gene goes home to the South for a brief vacation. On his way back to school, he stops by Finny’s house and explains to his friend that he shook the branch on purpose. Finny refuses to listen to him, and Gene rescinds his confession and continues on to school. There, Gene attempts to avoid true athletic activity by becoming assistant manager of the crew team, but he feuds with the crew manager and quits. World War II is in full swing and the boys at Devon are all eager to enlist in the military. Brinker Hadley, a prominent class politician, suggests to Gene that they enlist together, and Gene agrees. That night, however, he finds Finny has returned to school. He consequently abandons his plans to enlist, as does Brinker. Finny expects Gene to take his place as the school’s sports star now that he is injured. When Gene protests that sports no longer seem important in the midst of the war, Finny declares that the war is nothing but a conspiracy to keep young men from eclipsing the older authorities.
Finny tells Gene that he once had aspirations to go to the Olympics, and Gene agrees to train for the 1944 Olympics in his place. All the boys are surprised when a gentle, nature-loving boy named Leper Lepellier becomes the first one in their class to enlist. Gene and Finny go on training, shielded within their private vision of world events. During a winter carnival, which Finny has organized, a telegram arrives for Gene from Leper, saying that he has “escaped” and desperately needs Gene to come to his home in Vermont. Gene goes to Vermont and finds that Leper has gone slightly mad. Leper, who was present at Finny’s accident, reveals that he knows the truth about what happened. Leper’s ranting frightens Gene and makes him anxious about how he himself might react to military life. He runs away back to Devon. When Brinker hears of what has happened to Leper, he laments in front of Finny that Devon has already lost two of its potential soldiers—Leper and the crippled Finny. Gene, afraid that Finny will be hurt by this remark, tries to raise his spirits by getting him to discuss his conspiracy theory again, but Finny now denies the war only ironically.
Brinker, who has harbored suspicions that Gene might have been partly responsible for Finny’s accident, wants to prove or disprove them definitively. He organizes an after-hours tribunal of schoolboys and has Gene and Finny summoned without warning. The boys on the makeshift tribunal question the two about the circumstances surrounding the fall. Finny’s perceptions of the incident remain so blurred that he cannot speak conclusively on the matter; Gene maintains that he doesn’t remember the details of it. The boys now bring in Leper, who was sighted earlier in the day skulking about the bushes, and Leper begins to implicate Gene. Finny declares that he does not care about the facts and rushes out of the room. Hurrying on the stairs, he falls and breaks his leg again.
Gene sneaks over to the school’s infirmary that night to see Finny, who angrily sends him away. Gene wanders the campus until he falls asleep under the football stadium. The next morning, he goes to see Finny again, takes full blame for the tragedy, apologizes, and tries to explain that his action did not arise from hatred. Finny accepts these statements and the two are reconciled. Later, as the doctor is operating on Finny’s leg, some marrow detaches from the bone and enters Finny’s bloodstream, going directly to his heart and killing him. Gene receives the news with relative tranquility; he feels that he has become a part of Finny and will always be with him. The rest of the boys graduate and go off to enlist in relatively safe branches of the military. Gene reflects on the constant enmity that plagues the human heart—a curse from which he believes that only Finny was immune.