The story takes place 13 years ago, when suddenly one part of Tokyo broke off and began to float in the sky. Tokyo was separated into "Old Tokyo" as the part that was floating in the air, and "New Tokyo," the part that stayed on the ground. Communication and interaction between the two Tokyos was impossible, and years passed. One day, a young girl perceives that Old Tokyo will fall back down to the ground, and she sets out on the perilous path from from Old Tokyo to New Tokyo to save the citizens from the impending crisis.

[Source: Anime News Network]