Horizon on the Middle of Nowhere

In the far future, humans abandon the devastated Earth and move to the upper world "Tenjou." But Tenjou and the law of causation collapse by a war and humans returns to the Earth, which has turns into an inhabitable planet except for Shinshuu area. Shinshuu is too small to accommodate all the humans, so they duplicate the area and create "Juusou Sekai." In order to rebuild Tenjou and the law of causation, the returned humans start reproduction of history from B.C. 10,000. When the reproduction completes until A.D. 1413, a war breaks out in Shinshuu and Juusou Sekai falls onto the original world. The humans living in Juusou Sekai lose their land and invade the original world. People in Shinshuu surrender and is divided by the invaders from Juusou Sekai. They try to resume the history reproduction from A.D.1457, but the update of history terminates in A.D.1648. A rumor of apocalypse begins to spread around the world.