Takaki and Akari are two classmates in elementary school. During their time together they have become close friends. Their relationship is tested when Akari moves to another city because of her parents’ jobs. Both of them struggle to keep their friendship alive, as time and distance slowly pulls them apart. When Takaki finds out that he is moving further away, he decides to visit Akari one last time.

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.