Gene Forrester, a man in his mid thirties, describes his return visit to the private prep school he attended, the Devon School, in New Hampshire fifteen years earlier. There are two important places that Gene most wants to see. First he visits the "First Academy Building," one of the school's main buildings. Once inside, he feels older, taller, and more secure than he did as a student.
Gene's return to his prep school allows him to confront his adolescent past from a grown-up perspective. One change: as an adult he feels more secure with who he is than he did as a student.
As Gene looks around the building, he observes that it looks almost exactly the same as it used to. He observes that because it hasn't changed in 160 years, the Devon School allows past and present to mingle harmoniously.
Gene sees the Devon School's continuity as providing a harmonious relation with the past. The implication is that he doesn't feel that same harmony.
Gene's next important stop is a tree by a river. The tree still has a branch hanging over the river, but it looks smaller to him than it used to. Gene thinks to himself: the more things stay the same, the more they change. He finds this comforting, and describes himself "changed."
The tree, which hasn't changed, also isn't as big as Gene remembered it. The difference in his perception shows him how much he's changed, and seems to free him from a past that haunted him.
The story shifts to the distant past, with Gene recalling the Devon summer session of 1942, when he was sixteen and World War II was in full swing. Gene refers to it as his "sarcastic" summer.
As the world went to war, sarcasm, a classic teenage tool, became a way for Gene to deny his fear of the war.
Gene stands near a giant tree by a river with his best friend, Phineas ("Finny"), and three friends: Elwin "Leper" Lepellier, Chet Douglass, and Bobby Zane. Finny is about to jump from the tree branch hanging near the river, a physical test no boy his age has ever attempted. It's part of the fitness test that seniors at Devon have to pass before graduating. Finny refers to the jump as his "contribution to the war effort."
Finny's desire to jump from the tree despite his young age is the first hint of his immense athletic skill and adventurous spirit. Finny is also clearly the leader of the group. His comment about the "war effort" is not sarcastic; he conceives of war as just another game.
Finny climbs the tree and jumps. Gene is next. He climbs the tree. Though he's terrified, thinks jumping is stupid, and wonders if Finny may have gotten "some kind of hold" on him, Gene jumps. The other boys all refuse to jump. Finny tells Gene, "It's me and you, pal."
Gene jumps because Finny did, and his sense of rivalry makes him fear Finny has some kind of hold over him, over his identity. Finny, meanwhile, thinks they're just friends.
As the five boys walk back to dinner, Finny says that Gene did well after being "shamed" into it, and then makes fun of Gene for being so careful to arrive at dinner on time. Gene tackles Finny. They wrestle, and end up so late for dinner they decide to skip it. They go to their dorm room, do their homework, and go to sleep.
Gene and Finny have a real friendship. But Finny, comfortable in his athletic perfection, has no sense that his jokes actually hurt Gene. Gene, meanwhile, tackles Finny partly in fun and partly because the jokes anger him. It shames him that they're accurate.