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A Separate Peace, by John Knowles, is a novel told in first person by the narrator Gene Forrester. It is a story that is told in retrospect. Gene comes back to his private high school, Devon, after fifteen years have passed and attempts to come to terms with the events that happened there beginning with the summer before his junior year. The pivotal event is Finny's fall from the tree that resulted in the once gifted athlete being unable to participate fully in sports again. What Gene attempts to understand and accept is his role in Finny's fall. The story is an agonizing confrontation of the truth of Gene's relationship with Finny. Gene's jealousy of Finny quite overcame his friendship with him, and Gene did indeed jostle the limb that caused Finny's tragic fall. The resulting chapters track both Gene's and Finny's denial and final acceptance of the truth. This denial has much to do with their innocence, their inability to believe in the darkness of man's heart and their own imperfections. This innocence pervades their feelings about War World II, a truth that each boy evades as well.
This is only a short description of the story. Enotes has a thorough description of the plot developments, a link I posted below.