In a distant future, humans forsake a ruined Earth for space but are forced back by a mysterious barrier, finding only Japan habitable. To house the population, pocket dimensions arise around Japan. The people, following the Testament, reenact history to uncover space travel. Yet, in 1413 of the Testament Era, Japan is invaded, carved into fiefs, and its natives displaced. Fast forward to 1648 of the Testament Era, Japan's exiles dwell on the mobile city ship Musashi, surveilled by the Testament Union. As uncertainty looms with the Testament's silence post-1648, rumors of doom and war spread. Toori Aoi, leader of Musashi Ariadust Academy's Supreme Federation, seizes the chance to reclaim their lost land alongside his classmates.