Mood is the emotional atmosphere or quality within the story. Since A Separate Peace is a longer text, the mood shifts as the story develops. Much of the emotional quality of the story depends upon the central character, Gene, and his emotions and feelings influence the mood of the story. For example, the novel opens as Gene returns to Devon, and the mood of the opening chapter is decidedly grim and dark as he revisits areas of the school that feel haunted to him, particularly the tree.
Later in the novel, the mood grows increasingly dark as Gene's feelings of paranoia and insecurity consume him. Contrastingly, after Phineas' accident, the mood shifts as Gene's feelings of guilt, remorse, and confusion consume him. By the end of the novel as the boys go their separate ways to join World War II, the mood remains dark; only as Gene reminisces about Finny does the mood brighten to reflect his innocence and zeal for life.