A new subway line is laid under the city. But upon completion the tracks get altered by the magic sphere and start to lead to another world instead. The unimaginable magic force breaks free and destroys the whole city leaving it in devastation. Demons pour from the cracks in earth.
Government can't do anything with the polluted area, so a wall is built around it in 200 km radius.
Remaining citizens inside the area become trapped with the demons. But humans manage to adapt everywhere, so some humans learn to feed on dark creatures. Fluids and organs become traded at high prices, so the number of hunters arises.
Chris and Jean spend their days hunting for youkai. One day retired hunter shows them a picture of a demon. But the image shows only lovely twins...

[From vndbreview]