The book is written in three parts.
"Book One: The Romantic Egotist"—The novel centers on Amory Blaine, a young Midwesterner who, convinced that he has an exceptionally promising future, attends boarding school and later Princeton University. He leaves behind his eccentric mother Beatrice and befriends a close friend of hers, Monsignor Darcy. While at Princeton he goes back to Minneapolis, where he re-encounters Isabelle Borgé, a young lady whom he had met as a little boy, and starts a romantic relationship with her. At Princeton he repeatedly writes ever more flowery poems, but Amory and Isabelle become disenchanted with each after meeting again at his prom.
"Interlude"—Following their break-up, Amory is shipped overseas, to serve in the army in World War I. (Fitzgerald had been in the army himself, but the war ended while he was still stationed on Long Island.) Amory's experiences in the war are not described, other than to say later in the book that he was a bayonet instructor.