Twins, separated by fatal illness and a selective cure. Kasumi and her sister, Shizuku, were infected with the Medusa virus, which slowly turns the victim to stone. There is no cure, but of the two only Kasumi is selected to go into a sort of cryogenically frozen state along with 159 others until a cure is found. At some point in the undetermined future, Kasumi awakens to find herself and others who were in suspended animation in an unfamiliar world with violent monsters. Resolving to unlock the mysteries of her current situation and the fate of her twin sister, Kasumi struggles to survive in a treacherous world.