Hikari Sakishima and his childhood friends Manaka, Chisaki, and Kaname have always lived beneath the waves in the sea village of Shioshishio, protected by a skin-like membrane called Ena. When their underwater school shuts down, the four transfer to a surface middle school, where Hikari clashes with land-dwellers while Manaka befriends Tsumugu Kihara, a quiet boy fascinated by the sea. Tensions between the two communities mount as an ancient ritual meant to appease the Sea God approaches.