In the late 1950s, 15 years after graduation, Gene Forrester returns to Devon, an elite prep school in New Hampshire. Walking through the campus in the cold November mist, Gene remembers his experiences at Devon during World War II, especially the Summer Session of 1942, when he was 16 years old.
At a tree by the river, Gene thinks of his friend and roommate, Phineas (nicknamed Finny), the best athlete in the school. As the story moves into the past, Finny jumps from a high limb of the tree into the river — an activity forbidden to all but the oldest Devon boys — and dares Gene to jump as well. Gene jumps, but is frightened. Finny, however, takes such delight in the dangerous, forbidden jump that he forms the Suicide Society and invites all the Devon boys to test their courage by jumping from the tree into the river. At each initiation, Gene and Finny make the first jump, but Gene never gets over his fear.
Finny's status as the best athlete inspires Gene to strive to become the best student in the school. Gene applies himself to his studies seriously, but feels pressure from Finny to join in his activities, especially the Suicide Society. After failing a math test because of a forbidden trip to the beach, Gene suspects that Finny is deliberately trying to sabotage his studying. This silent resentment builds until the end of the summer, when Finny insists that Gene leave his books to jump from the tree again. High in the tree with his friend, Gene impulsively jounces the limb and causes Finny to fall.
