A Separate Peace begins when Gene returns to New Hampshire's Devon School and reflects on his time there with particular attention to his roommate Phineas. Gene recalls how innocent and charming Finny was. There was a time when Gene failed a test, thinking that Finny (by getting him to miss studying time) was trying to sabotage him. Gene later finds out that Finny supposed Gene was so smart he didn't need to study. Gene realizes that Finny never had bad intentions.
The most significant event in the novel is when Gene pushes (inadvertently or purposefully) Finny out of the tree. This results in a broken leg and eventually, after a botched procedure, leads to Finny's death.
Back at school, one night in the Assembly room, Brinker and some other students confront Gene about his possible guilt in the accident. Finny can't take it, leaves the room and subsequently breaks his leg again. Finny dies shortly afterwards. And Gene realizes that he never really hated Finny. He was jealous of Finny's innocence. At a time when these boys were on the threshold of adulthood, a process sped up by their potential to be sent to war, Gene resented how Finny could avoid the pressures and responsibilities of growing up.