Four years have elapsed since the initial outbreak of the Unknown virus epidemic, and the terrifying plague that depopulated Japan's Tohoku region has faded into a distant memory, owing to strict quarantine measures combined with an effective vaccine. However, the Unknown virus is not yet extinct, and an inescapable doom will soon, very soon, befall Wataru Ayase along with his family and friends, confining them within the "Hell on Earth" asylum that was once their home city of Tokyo...