In a post-war alternative timeline, Japan is divided into the North, controlled by the Union, and the South, controlled by the United States. A mysterious high tower rises within the borders of the Union. Three high school students promise to cross the border with a self-built airplane and unravel the secret of the tower.

Three moments in Takaki's life: his relationship with Akari and their forced separation; his friendship with Kanae, who is secretly in love with him; the demands and disappointments of adulthood, an unhappy life in a cold city.