Ruka Azumi's ordinary summer vacation revolves around spending all her time on the handball court to avoid her only housemate, her alcoholic mother. When an incident at practice leads to Ruka's exclusion from the handball team, she is forced to find other outlets to enjoy herself, eventually winding up at the aquarium where her father works. There, she meets Umi, a mysterious boy raised by dugongs who is being kept at the aquarium in order to research his strange water-dependent body.

Ruka and Umi become fast friends, and the pair is soon joined by Sora, Umi's brother who also has an unusual relationship with the ocean. Together, the trio explore a dazzling undersea world while learning about a peculiar "festival" that all sea creatures around the world are preparing for.