Following Japan's defeat in World War II, the country was divided between two powers: Hokkaido came under the control of "Union," while Honshu and other southern islands fell under the sovereignty of the United States. A towering structure of unknown origin was erected in Hokkaido, visible from Honshu's northernmost province, Aomori, separated by the Tsugaru Strait. In the summer of 1996, three ninth-grade students made a pact to construct an aircraft to investigate the tower, but Sayuri Sawatari, one of the trio, fell ill with a severe sleeping disorder and had to move to Tokyo for medical treatment. Despondent, the project was abandoned. Three years later, Hiroki Fujisawa discovered that Sayuri had been in a comatose state since she left; determined to find a cure, he enlisted the help of Takuya Shirakawa. Little did they realize that Sayuri's condition was linked to the secrets of the mysterious tower and the wider world.