More than 200 years ago, a fleet of immigrants drifting in space landed on this planet. Human beings once again set foot on a long-lost land. Civilizations from the earth have rapidly multiplied and revived on this ancient land. When people became accustomed to and even considered themselves the masters of the planet, the "black tide" spread out from the depths of the earth. It does not have a physical form, but it actually exists. It comes like a natural disaster, destroying the city and tearing the ground. Neither humans nor data can escape its infection and erosion; it also seems to have its own consciousness. The living beings, chasing the traces of civilization, forced mankind from the earth back to the island-like coordinate cities. In order to deal with the threat of erosion tide, Libit, a giant enterprise in Urania, the No. 9 coordinate city, established a special security countermeasure agency - Nightingale. And you will lead the Nightingale to organize girls with various types of wing-carved bodies to investigate various events caused by the erosion tide, and explore the truth behind the natural disaster.