Ryugu is a small town built on the sea floor, an underwater community that outwardly resembles any ordinary small town. Middle school classmates Masora and Sumire live a seemingly normal life, swapping stories about crushes and spending time together at the local shopping mall. But beneath the surface of this routine existence, something feels off. Masora becomes troubled by the town’s strict control over information, and she begins to investigate on her own. Meanwhile, Sumire carries a vague but growing sense that their peaceful, everyday life is about to come crashing down. As Masora digs deeper, she encounters suspicious elements—shadowy groups and potential terror threats—that suggest the underwater town hides secrets far more unsettling than its unusual location. The story follows the two girls as their ordinary days give way to a slow-building tension between curiosity, danger, and the fragile nature of their world under the sea.