Living in a rural town where only a dozen or so trains run daily, our protagonist leads a carefree life on his own, despite being occasionally pushed around by his eccentric father, who travels abroad for work.
Then, in a sudden phone call from his father, he learns that he has a new "stepsister." What's more, this out-of-nowhere stepsister will be arriving at the nearest station in about five minutes. Rushing to pick her up, unable to organize his thoughts, our protagonist meets a girl there.
"I'm Fuzuki Sora. Nice to meet you."
The girl has blonde hair that flows in the sunlight and captivatingly clear blue eyes, but she bows her head in a modest manner that doesn't suit her appearance.
Confused and awkward, the two head home.

In a corner of the vast sky, the story of a certain brother and sister begins...