Humanity Has Declined takes place in a post-apocalyptic world where the human population is steadily declining. The most prosperous species on Earth are Fairies. A nameless girl returns to her hometown to take over her grandfather's position as arbitrator between humans and fairies. When the village is sent some strange products, the girl and her grandfather investigate who is behind making them. What starts out as a small task turns into the most important mission facing the human race!