If we're to look at the sheer facts of this story, there really isn't that much to it: an American soldier is in a top secret training program in England, where he meets a rather strange and extraordinary pair of siblings, thirteen-year-old Esmé and five-year-old Charles. He chats with Esmé somewhat superficially about the war, her deceased parents, and her plans for the future, and he kids around with the rather manic Charles. They part ways after exchanging addresses, and Esmé wishes him luck in the war (World War II).
Months later, Sergeant X (the narrator, "cunningly disguised") is recovering from the war; he's on the brink of a nervous breakdown, and can't seem to get a grip on reality. His comrade, Clay, tries unsuccessfully to talk him out of his depression. Upon receiving a letter from Esmé, however, Sergeant X feels a kind of peace settle upon him, and finally is able to relax – her letter gives him the sense that he might just be OK after all.