Kyoukai Senjou no Horizon

When humans came down from the sky they brought the Testament, the guide to the path they must follow if they wish to return to the skies. Now, in a world where only the islands of Japan are inhabitable, the nations of the world vie for power. But the Testament ends abruptly, and it is now the last year. Is this the end of humanity, or can Tori Aoi and his fellow students somehow affect the course of destiny? Who and what is the Horizon in the Middle of Nowhere?