The sky city of Utopa serves as a refuge for endangered species that were relocated there after the devastation of the Earth. Over time, these species evolved and developed a civilization of their own, including language and bipedalism, through artificial genetic crossbreeding. However, their isolated existence has led them to forget about the world below. When a plant seed inadvertently enters the city, three children - Kui, Hiruma, and Root - discover it, sparking their curiosity about the unknown world outside.
