When Natsu Iwashimizu awakens from a dream of cherished family moments to a dire situation on a sinking ship, she is rescued by three strangers: the contradictory Arashi Aota, the mocking Semimaru Asai, and the capable Botan Saotome. Their escape leads them to a mysterious island that turns out to be post-apocalyptic Japan, transformed by a cataclysmic event. As part of the government's '7 Seeds' project aimed at repopulating the Earth, Natsu and her companions are part of team Summer B, labeled societal failures. To survive in this harsh new reality, they must locate government shelters while navigating a world devoid of familiar comforts, relying solely on each other for support.