From AnimeNewsNetwork:

The story is set several centuries into the slow decline of mankind. Earth has now become the realm of "fairies" - 10-centimeter-tall (about 4-inch-tall) creatures with remarkable intelligence. The human protagonist of the story serves an important duty as the international arbitrator between the fairies and humans, and she returns to her former hometown of Kusunoki. She chose this job because she thought it was an easy job she can do, even when she is as old as her grandfather. However, she learns that her job is anything but simple when she meets the fairies.