Natsu Iwashimizu's last memory of her family is eating her favorite meals with them. Unfortunately, she wakes up from her dream only to find herself in a sinking ship, being rescued by three strangers: Arashi Aota, whose kind-hearted personality does not match his quick temper; Semimaru Asai, who enjoys ridiculing Natsu's meek nature; and Botan Saotome, a skillful woman seemingly aware of their situation. Following the four's successful escape, they sail to a deserted island.

Unbeknownst to them, the mysterious island is Japan. After a meteorite landed on Earth, humanity was wiped out and the environment changed drastically. To repopulate the country, the government initiated the "7 Seeds" project, consisting of five groups—Spring, Summer A, Summer B, Autumn, and Winter—each with seven young healthy individuals and a guide. Natsu and the others were put into team Summer B, whose members are considered failures by society.

To get more supplies, the group must find one of the seven shelters built by the government. In a hostile, unfamiliar world with no loved ones to depend on, the quartet must learn to cooperate with each other to ensure their survival.