The last thing Natsu Iwashimizu remembers about her family is sharing her favorite dishes with them. But when she wakes from that dream, she finds herself aboard a sinking ship, rescued by three strangers: Arashi Aota, a kind-hearted person with a quick temper; Semimaru Asai, who enjoys mocking Natsu's timid personality; and Botan Saotome, a capable woman who seems to understand what is happening. After escaping, the four sail to an uninhabited island.
Unknown to them, this mysterious island is actually Japan. A meteorite struck Earth, wiping out humanity and drastically altering the environment. To restart the population, the government launched the "7 Seeds" project, dividing people into five teams—Spring, Summer A, Summer B, Autumn, and Winter—each composed of seven healthy young individuals and a guide. Natsu and her companions are part of Summer B, a group labeled as societal misfits.
To secure more resources, the group must locate one of seven government-built shelters. In a hostile, unfamiliar world without loved ones to rely on, the four must learn to cooperate if they want to survive.