Unknown enemy forces have been waging a desperate war for five years, and the inhabitants of remote island Aoshima remained largely unaware of the terrible conditions on the mainland. But everything changes abruptly when scholars are handpicked to serve on the frontlines every Friday at the fifth hour. Despite being deemed unsuitable for service due to undisclosed reasons, Saku and Miyako are excused. Conversely, Reina, a young escapee from Tokyo, is sent immediately to the combat zone.