Based on the erotic game by Speed.

Four years have passed since the initial outbreak of the Unknown virus epidemic, and the terrifying plague which depopulated the Tohoku region of Japan has been reduced to a mere memory, thanks to drastic quarantine measures coupled with an effective vaccine.

But the Unknown virus is not dead yet and soon, very soon, an unescapable doom will fall upon Wataru Ayase and his family and friends, trapping them inside the "Hell on Earth" madhouse which originally was their home city, Tokyo...

(Source: Visual Novel Database)