Based on the short story of the same name.

This story occurs before the events of the war that brought the world to ruin; at this point, Yumemi has been working at the Flowercrest Department Store's rooftop planetarium for about ten years. One day, Yumemi begins to act strangely, culminating in her simply walking out and wandering around the town. The staff of the planetarium are bewildered, and one of the workers—a woman named Satomi Kurahashi—is ordered to go follow Yumemi and bring her back. Before long, Yumemi begins to run out of battery power.

[Taken from English Wikipedia]