The main character, Gene Forrester returns to visit the campus of Devon School, an all-boys elementary school in rural New Hampshire, which he attended fifteen years ago. Two locations have a mysterious significance to him, the first being a marble staircase inside a classroom building. The second is beneath a tree growing near Devon River. After standing outside, time moves back fifteen years to when Gene had still been a schoolboy standing near that same tree. His friend Phineas urges him to climb its branches and jump off to simulate abandoning a torpedoed ship, as World War II is waging on in the outside world. Gene jumps out of its branches into the river with Finny, despite his fear.

During an accelerated class session in the summer of 1942, Gene becomes increasingly paranoid of Finny who is also his roommate. He is the best at every sort of sport...