Yukito doesn’t have a single relative after a great flood which claimed the lives of his parents when he was small. That was also when his younger sister went missing. One day, he happened to hear in the news that the past records pertaining to the disaster would be available to the public for the 15th anniversary of the event. He decided to return to his hometown of Kugumi village, holding on to the belief that if he checked those records he might be able to reunite with his long lost younger sister that he only faintly remembers.
Source: Hau~ Omochikaeri~!