On an otherwise ordinary day, Minakami Yuki watches a worn and battered stuffed toy fall from the sky — not from a building, but simply from the air, as if dropped from somewhere high above the clouds. The image stays with her: the doll is an offering, she learns, part of a local ritual passed down for generations. The ceremony is meant to find "the day we return to the sky" — a phrase that means more the deeper into the story she goes.
The story centers on the Summer Triangle: Vega, Altair, and Deneb — or in local terms, Orihime, Hikoboshi, and the Northern Cross — three figures separated across a vast distance. Three students, Takashima Zakuro, Wakatsuki Kagami, and Wakatsuki Tsukasa, alongside Yuki herself, follow the thread of the ritual toward the place where the heavenly triangle's points converge — and where the boundary between the world below and the sky above grows thin.