Hiroki Fujisawa and Takuya Shirakawa harbor admiration for two things in their life: their classmate Sayuri Sawatari and the vast Ezo Tower that stands boundlessly across the Tsugaru Strait. Fascinated by the limitless structure beyond their reach, Hiroki and Takuya begin constructing an aeroplane from a fallen drone they discovered—naming it the Bella Ciela—to fulfill their dream of one day reaching the sky-scraping top of the tower. Later joined by the girl they love, Hiroki and Takuya promise Sayuri to fly with her to the seemingly otherworldly top together. However, Japan has suffered a North-South partitioning that has fueled conflict near the base of the tower, which marks the border between the America-controlled Southern islands and the Northern lands occupied by the Soviet Union. Further along, Sayuri suddenly disappears, and Hiroki and Takuya never see her again. Unbeknownst to them, she fell victim to a sleeping disorder that left her comatose for the past three years. Although Hiroki and Takuya later learn about Sayuri's condition, they also discover that the girl's unconscious state is oddly linked to the same tower the trio had promised to conquer together. In an alternate history of the aftermath of World War II, Japan has been cleaved in half, with the south — Honshu and the other islands — allied with the United States and the northern island Hokkaido annexed by the enigmatic Union. It was on Hokkaido that a mysterious tower had been built, a strand of metal reaching up out of the atmosphere, clearly visible from the northern tip of Honshu.In 1996, three teenagers, Hiroki, Takuya and Sayuri, make a pact — they will build an experimental aircraft, almost invisible to surveillance, cross over to Hokkaido and unlock the secrets of the tower. Their dream was never realized, because Sayuri was sent to Tokyo for treatment after she fell into a coma.It is now three years later. The rosy illusions of youth have fallen away, but not the unbreakable strength of the trio's promise. The truth of the tower will be uncovered, and with it, the link between it and Sayuri's mysterious, carefully tended condition.