偶遇到神秘少年“海”（石桥阳彩配）和“空”（浦上晟周配）的这个炎炎夏天，是琉花（芦田爱菜配）暑期的正式开始。天真无邪的“海”，看透一切的“空”，海洋馆里发光的鱼群，对琉花来说都像是一场浪漫的约定如期而至。Ruka is a young girl whose parents are separated and whose father works in an aquarium. When two boys, Umi and Sora, who were raised in the sea by dugongs, are brought to the aquarium, Ruka feels drawn to them and begins to realize that she has the same sort of supernatural connection to the ocean that they do. Umi and Sora's special power seems to be connected to strange events that have been occurring more and more frequently, such as the appearance of sea creatures far from their home territory and the disappearance of aquarium animals around the world.