Centuries after the 20th century, nature and the ecosystem have been devastated by the use of nuclear and biological weapons. Most of the Earth is contaminated and collapsing into ruin, with the vast majority of plants and animals now extinct. The world is heading toward the "Big Six," the sixth mass extinction event. Humanity constructed cities in the sky, relocating endangered species of animals and plants there. Over many years, the animals evolved within these isolated cities. Through artificial genetic crossbreeding, they acquired language and bipedality, building a life and civilization for themselves free from want. No one thinks of the world below anymore. One day, a single plant seed wanders into one of the sky cities, where the children Kui, Hiruma, and Root discover it. Unable to resist the temptation to look into the outside world, they decide to undertake an adventure there, unaware that once they set foot on the ground, they will never be able to return.