Cut off from her summer club after lashing out at a teammate, middle-schooler Ruka Azumi drifts to the aquarium where her father works and meets Umi and Sora, two boys raised by dugongs and strangely at home in the water. Scientists puzzle over where the brothers came from while fish vanish from oceans around the world. Pulled into the boys' orbit, Ruka starts hearing the same calls from the sea that they do.