Jinrui wa Suitai Shimashita: Yousei, Shimasu ka?

Humanity has seen a significant decline over the centuries due to a sharp decrease in birthrates, leading to the rise of Fairies as the dominant species. These small, 10 cm tall beings possess high intelligence. The protagonist works as a UN mediator between humans and Fairies and returns to their hometown to assist their grandfather. Assuming the role would be manageable, they visit a Fairy habitat to greet them, only to encounter an unexpected situation.