After an asteroid impact in 1999, the world collapsed. Rui Yoichi, the founder of the Ten-no-Shizuku order, foresaw the catastrophe and saved a small group of people by leading them underground. Fifty years later and eight years after his death, this small world that had lost its leader is in danger of falling into chaos. Hotaru, his childhood friend Hikari and his admirer Asuka live in District N, the fourth and most degenerate underground district, constantly fighting with dealers that Hotaru hunts down. Meanwhile, the government adopts the "N18 Plan," which aims to eradicate the N District. Disillusioned by this inhumanity, Rui's son and government member Ashidaka mingles with the people and befriends Hotaru. But then there is the mysterious blue mask in whose possession Hotaru is. What will become of District N and Hotaru's dream of seeing the surface world?