One day, a UFO invaded Earth. For several years, numerous aliens had been exterminated, but this time was a nightmare. The highest-ranked Class R creatures devoured every living thing in sight, and humans were no exception. The Alien Extermination Squad, including Guest, managed to kill what was presumed to be the mother, but the losses were severe. In the end, Guest was the only survivor. While walking out of the damp cave to return to headquarters, the sound of eggs cracking echoed from somewhere. The sound didn't stop. Dozens of eggs cracked in various places, and the things the mother had been protecting emerged into the world all at once.