The manga is 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 only attack people. 13 years after that "Red Day" of societal and civilizational collapse, the surviving humans survive away from the big cities, while still hoping to retake them someday. An entirely new generation of people who know nothing of the pre-Red Day world are now coming into their own. The story begins when Aki and her close friends Haru and Natsuki begin a journey to find Aki's missing father.

(Source: Anime News Network)