The manga takes place in a world where 90 percent of Earth's human population contracted and died from a deadly, virulent disease, only to rise again as zombies that attack people. Thirteen years after that "Red Day" of societal and civilizational collapse, surviving humans live away from major cities, still hoping to reclaim them someday. A completely new generation, knowing nothing of the pre-Red Day world, is now coming of age. The story begins when Aki and her close friends Haru and Natsuki set out on a journey to find Aki's missing father.