April 16, 2056. Five years ago, the disaster struck. The government had determined that the surface was no longer habitable due to pollution, and was in the process of planning and building marine cities for relocation.

The catastrophe began with a miscalculation by the construction firm. They decided to build the city on a highly seismic seabed, and the result was a magnitude 7.0 earthquake.

The sea level rose rapidly, submerging everything up to the height of a 20-story apartment building. The number of casualties was beyond counting.

...If only it had ended there. Due to the massive undersea earthquake and the surge in water levels, radiation research labs were all destroyed, exposing humans and fish alike to radiation.

Over 80% of humanity perished. The fish fared no better. The surviving organisms were no longer what they once were.

The physical abilities of the surviving humans increased abnormally and they even developed gills, forcing them into the underwater life they had only dreamed of before the disaster.

The fish gained intelligence through radiation exposure. Humanity called these creatures 'Demonfish'—hostile fish that easily exceeded 2 meters in size.

As usual, I was walking through the deserted streets toward a grocery store, a weapon for dealing with Demonfish slung across my back. Suddenly, I heard a woman's scream in the distance.