In a far-off era, mankind escapes a devastated Earth and sets out to explore the vast expanse of space. Unfortunately, their journey is cut short by an impenetrable barrier that forces them to turn back to their ruined planet. Surprisingly, Japan survives in the midst of the disaster, leading to the creation of pocket dimensions to hold the growing population. As they try to unravel the reasons behind their calamity, the people seek guidance from the Holy text, known as the Testament, to help them leave Earth for good. However, their plans are hindered by attacks from neighboring realms, causing the locals to become estranged from one another. Fast forward to 1648, and the refugees from Japan reside in the mobile city-ship, Musashi. Governed by the Testament Union, the residents hold on to hope despite rumors of impending danger. Toori Aoi, the leader of the Musashi Ariadust Academy's Supreme Federation and President of the student council, takes advantage of the situation to gather his fellow students and embark on a daring quest to reclaim their lost homeland.