Future. A world divided into sky and earth. The surface has been sealed off as a forbidden land, swallowed by an all-consuming gel-like substance and forests of colossal trees. Humanity is confined to the "Suspended Cities" built in the heavens, and these city clusters now face the crisis of collapse. Two boys, one born and raised in the uppermost layer of the city and the other in the lowest, had lost their place to live within society. These two meet and descend to the forbidden land. In that place, which should have been a forest of death, they encounter a girl. The boys journey with the girl, meeting various people and creatures. The seemingly primitive forest people possess a civilization more advanced than that of the sky dwellers. The two delve into the mysteries of the forest and its people. And further, into the reason why the world was split into earth and sky. And in doing so, they discover their own way of life and the potential of humanity.