4 years have passed considering that the initial outbreak in the Unknown virus epidemic, as well as the terrifying plague which depopulated the Tohoku region of Japan has been reduced with a mere memory, because of drastic quarantine measures coupled with a powerful vaccine.

Nevertheless the Unknown virus just isn't dead yet and very quickly, very soon, an inescapable doom will fall upon Wataru Ayase with his fantastic friends, trapping them in the "Hell on Earth" madhouse which originally was their residence city, Tokyo...