In this manga, humanity faces a deadly disease that has killed over 90% of people worldwide. Those who survive become zombies that attack others. Thirteen years after this "Red Day," which marked the collapse of society and civilization, remaining humans live outside cities, hoping to one day return. New generations are growing up without knowledge of the past world. The story follows Aki and her friends Haru and Natsuki as they embark on a journey to find Aki's missing father.