Ruka Azumi spends her typical summer break glued to the handball field, using it as a shield from her single roommate, an alcoholic mother. After a mishap during training forces Ruka off the squad, she must seek fresh pleasures and eventually finds herself at her father's aquarium job. There she encounters Umi, a strange lad raised by dugongs who is confined to the facility for scientists studying his bizarre aquatic physiology. Ruka and Umi quickly bond, and soon Sora—Umi's brother who also shares an odd affinity for the sea—joins them. Together, they dive into a vibrant underwater realm while discovering a strange "festival" that every marine creature worldwide is gearing up for. This mysterious celebration promises to unite species and reveal hidden currents of unity beneath the waves for everyone.
