Centuries after the twentieth century, Earth"s flora and fauna were shattered by nuclear and biological warfare. The planet became heavily polluted, crumbling into desolation, with most plant and animal life lost to extinction. Humanity approached the "Big Six," the sixth global mass‑extinction crisis. To preserve endangered species, people erected skyward cities and transported the remaining flora and fauna into them. Their city, a marvel of glass and steel, floated above the shattered continents. Over countless generations, those creatures adapted within these isolated habitats; through engineered genetic recombination they gained speech, walked upright, and forged a self‑sufficient civilization. The inhabitants no longer considered the earth below. One day, a lone seed drifted into one of these aerial metropolises, and the youngsters Kui, Hiruma, and Root found it. Drawn by curiosity to see beyond their lofty realm, they resolved to venture into the outside world, unaware that stepping onto the ground would make return impossible. They had never seen the sky's edge, nor heard of rivers or mountains. The seed"s arrival sparked a quiet rebellion in their hearts.
