Set in a world where 90 percent of humans on Earth have contracted and died from a deadly, virulent disease, only to then rise again as zombies who attack people. 13 years after that "Red Day" of societal and civilizational collapse, the surviving humans live outside the big cities, still hoping to retake them someday. An entirely new generation of people who know nothing of the pre-Red Day world is now coming into its own. The story begins when Aki and her close friends, Haru and Natsuki, set out to find Aki's missing father.