Within the safety of the walls, youths are raised in a nursery-style setting by robots. While life there may appear stale on the surface, the children are full of potential and curiosity. In many ways it is like a slice of heaven.
The outside world is a hell-scape. It is almost entirely void of anything mechanical and is now inhabited by bizarre, yet powerful super-natural beings.
Maru, with the aid of Kiruko, is out there crisscrossing what was once Tokyo for heaven. But after searching for so long, maybe heaven is more of an untenable dream than a potential reality.