Fifteen years after a catastrophic event, a group of children live in a secure facility, unaware of the outside world. One child, Tokio, receives a mysterious message asking if she wants to leave the facility. Another child, Mimihime, warns Tokio that two strangers will arrive, including someone who looks like her, and that the outside world is dangerous.

The desolate landscape of Japan, scarred by the Great Disaster, serves as the...