Kyoukaisenjou no Horizon

In a distant future, Earth becomes uninhabitable except for Japan, prompting humanity to return after being unable to leave due to an unknown space phenomenon. To house the global population, pocket dimensions are constructed around Japan. Humanity attempts to restore space travel by reenacting history based on the Testament, a holy book. By 1413 of the Testament Era, these nations invade Japan, dividing it into feudal domains and displacing its original inhabitants. In 1648, the displaced Japanese live aboard the city ship Musashi, which travels Japan under the oversight of the Testament Union. As the Testament ceases to provide guidance beyond 1648, rumors of an impending apocalypse arise. Toori Aoi, leader of the Supreme Federation and student council president at Musashi Ariadust Academy, organizes his classmates to reclaim their homeland.