In an alternate timeline, Japan was divided after World War II, with Hokkaido annexed by the Union and the southern islands under US sovereignty. A colossal and enigmatic tower was constructed in Hokkaido, visible from the northernmost prefecture of Honshu, Aomori, across the Tsugaru Strait. In 1996, three 9th-grade friends made a childhood promise to build an aircraft and unravel the tower's secrets, but their project was abandoned when one of them, Sayuri Sawatari, fell into a deep sleep and was transferred to Tokyo for treatment. Three years later, her friend Hiroki Fujisawa discovers that Sayuri has been in a coma ever since, and enlists the help of Takuya Shirakawa to revive her, unknowingly linking her unconsciousness to the mysteries of the tower and the world.
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