Charlie is a shy teenager without friends that has just joined the high-school. He misses his best friend, who had committed suicide, and he writes letters to an imaginary friend telling his feelings. Further, Charlie has a mental illness problem in his past. Soon Charlie befriends the veterans Patrick, who is gay, and his stepsister Sam and they become best friends. Charlie wants to be a writer and he also becomes the favorite student of his teacher of literature, Mr. Anderson, who lends him books. Sam and Patrick introduce Charlie to their friends and Charlie falls in love with Sam, but he does not have self-confidence to date her. Charlie discovers a new world and feels happy with his friends, and he dates their common friend Mary Elizabeth. But when they end their relationship, Patrick asks Charlie to stay away from them for a while and soon he triggers his mental illness again. He goes to the hospital where a doctor finally discloses the origin of his problem.