During her uneventful summer break, Ruka Azumi's routine of handball is shattered when she is ousted from the team. Seeking solace, she stumbles upon an aquarium managed by her father, where she encounters Umi, a peculiar boy nurtured by dugongs. As their bond deepens, they are later joined by Sora, Umi's brother with his own oceanic secrets. Their trio embarks on underwater adventures, discovering a global sea creatures' 'festival' shrouded in mystery.