Willem Kmetsch risks everything, leaving his loved ones behind to fight a mysterious monster. Though he emerges victorious, he ends up frozen in ice. During his long slumber, terrifying creatures called "Beasts" appear on Earth, threatening humanity's existence. When Willem awakens 500 years later, he discovers he is the last surviving human—his entire race has been wiped out.
Alongside the other remaining races, Willem seeks refuge on floating islands in the sky, living in constant fear of the Beasts below. He leads a solitary life, taking odd jobs just to get by. One day, he is assigned to serve as a caretaker for a weapon storehouse. Thinking nothing of it, Willem accepts—only to realize these weapons are actually a group of young Leprechauns. Though they look just like humans, they have no regard for their own lives, seeing themselves as mere tools of war. Among them is Chtholly Nota Seniorious, who is ready to sacrifice herself if it means defeating the Beasts and achieving peace.
Willem becomes a father figure to the young Leprechauns, watching over them fondly and supporting them however he can. He, who once fought bravely on the front lines, can now only hope that those sent into battle return safely from the monsters that destroyed his kind.