Shortly before Christmas, Meaulnes leaves the school in the village of Saint Agathe where he and François are students on an errand but does not return when expected. Meaulnes mysteriously reappears after three days, telling nobody where he has been. Meaulnes is not sure himself, and draws a map to try to work out where he has been. Eventually, Meaulnes relates the story of his journey to François, explaining about a strange estate he visited where he joined a dream-like engagement party for Frantz de Galais, the son of the owner of the manor and his fiancée Valentine, neither of whom were present. At the estate Meaulnes had encountered a young woman, Yvonee de Galais with whom he was enraptured. Later, Frantz had returned to the manor and announced that Valentine would not be coming to the engagement celebrations.
François and Meaulnes try to find the manor from Meaulnes' incomplete map but do not succeed until one night they encounter a gypsy and some children who steal Meaulnes' map. The next day, the gypsy joins the school and returns the map to the manor to Meaulnes, having filled in the missing details that will lead Meaulnes to the estate. François, Meaulnes and the gypsy become friends and Meaulnes swears to gypsy that he will come if the gypsy calls him. In return, the gypsy gives Meaulnes the address of Yvonne de Galais in Paris.
The gypsy reveals himself as Frantz de Galais when he is forced to flee the village, pursued by the police. Meaulnes despairs of ever finding the mysterious estate and departs for Paris in search of Yvonne. From Paris, Meaulnes writes to François, telling him the Yvonne is married.
Eighteen months pass and François, still in Saint Agathe happens to discover the location of the estate, in Old Nançay, that Meaulnes had been searching for. Via an uncle who lives in the village, François also finds out that Yvonne is in fact not married. François and his uncle arrange for Meaulnes and Yvonne to meet. When they are finally reunited, Meaulnes learns that the family had been forced to sell their estate to pay debts that Frantz de Galais had accumulated. Meaulnes realises that the magic of his first meeting with Yvonne is lost forever but, overcome with nostalgia, asks Yvonne to marry him.
On the day of his marriage to Yvonne, Meaulnes hears a cry - it is Frantz who is desparate to find his lost fiancée, Valentine. Meaulnes agress to honour his oath to Frantz and leave the next day with him to search for Valentine; Yvonne is left alone. Months later, Meaulnes has still not returned and Yvonne gives birth to a daughter but dies shortly afterwards, never having seen Meaulnes since their wedding day. François find Meaulnes' diary and learns that when Meaulnes was in Paris he had met and seduced Valentine. When Meaulnes discovered that she was Frantz's fiancée, he was wracked with guilt so that when Frantz returned to ask for his aid, Meaulnes swore to himself that he would not return to Yvonne unless Frantz and Valentine were reunited and married.
A year after François finds the diary, Meaulnes returns with Frantz and Valentine, finally married. He takes his daughter and disappears once more.