One day, a UFO invaded Earth. For several years, various aliens had been defeated, but this time was a nightmare. The R-class, the highest grade, devoured every living thing in sight, and humans were no exception. The Alien Disposal Unit, including Guest, managed to kill what was presumed to be the mother, but the losses were heavy. In the end, Guest was the sole survivor. While walking out of the damp cave to return to headquarters, the sound of eggs hatching echoed in succession. Dozens of eggs cracked everywhere, and the creatures the mother had been protecting emerged into the world all at once.