The focal point of Ruka Azumi's unremarkable summertime break involves her devoting every moment to handball practice, as she strives to avoid encountering her sole housemate, her alcoholic mother. Regrettably, Ruka is disqualified from the handball squad following a mishap during a training session, which prompts her to search for new sources of amusement. This leads her to the aquarium where her father is employed, where she crosses paths with Umi, a puzzling young man who was nurtured by dugongs and is presently being studied at the aquarium owing to his bizarre water-reliant physique. Ruka and Umi immediately click and develop a strong bond, with Sora, Umi's brother, who shares his sibling's unique relationship with the ocean, subsequently joining them. Together, the trio embarks on a mesmerizing aquatic adventure while simultaneously discovering a unique "festival" that is being prepared for by marine creatures worldwide.