François Seurel, the 15-year-old narrator of the book, is the son of M. Seurel, who is the director of the mixed-ages school in a small village in the Sologne, a region of lakes and sandy forests in the heartland of France. François is intrigued when 17-year-old Augustin Meaulnes, a bright young man from a modest background, arrives at the school. Because of his height, Augustin acquires the nickname "grand" ("tall"). He becomes a hero figure to the class and runs away one evening on an escapade where, after getting lost, he chances on a magical costume party where he is enchanted by the girl of his dreams, Yvonne de Galais, a character inspired by the real-life Yvonne de Quiévrecourt.[3] She lives with her widowed father and her somewhat odd brother Frantz in a vast and ancient family château, Les Sablonnières, which has seen better days. The party was being held to welcome Frantz and the girl he was to marry, Valentine. However, when she does not appear, Frantz attempts suicide but fails.
After returning to school, Meaulnes has only one idea: to again find the mysterious château and the girl with whom he has now fallen in love. However, his local searches fail while at the same time a bizarre young man shows up at the school. It is Frantz de Galais under a different name, trying to escape the pain of having been rejected. Frantz, Meaulnes, and François become friends, and Frantz gives Meaulnes the address of a house in Paris where he says Meaulnes will find his sister, Yvonne de Galais. Meaulnes leaves for Paris only to learn no one lives in the house anymore. He writes to his friend François: "...it is better to forget me. It would be better to forget everything".
François, who has now become a school teacher like his father, finally manages to find Yvonne de Galais and reunites her with Meaulnes. Yvonne still lives with her aging father in what is left of les Sablonnières, which is closer than the two young friends had first imagined in earlier years. Yvonne is still single and confesses to Meaulnes that he is and has always been the love of her life. She accepts, with her father's blessings, Meaulnes' marriage proposal. However, the restless Meaulnes leaves Yvonne the day after their wedding in order to find her lost brother Frantz (whom he had once promised to help) and re-unite him with his fiancée, Valentine. Yvonne remains at the château, where she gives birth to a little girl but dies two days later.
Eventually François lives in the house Meaulnes and Yvonne lived in and raises the little girl there, while waiting for the return of his friend Meaulnes. While looking through old papers, François discovers a handwritten diary by Meaulnes. During the years in Paris (before François brought Meaulnes and Yvonne back together), Meaulnes had met and romanced Valentine, the fiancée who had jilted Frantz on the night of the party.
Meaulnes does return, after a year and eight months, having brought Frantz and Valentine back together. He discovers that Yvonne has died and left a daughter, whom he claims. Four years have elapsed since the beginning of the story.
