Cut off from her summer club after lashing out at a teammate, middle-schooler Ruka Azumi wanders to the aquarium where her father works and meets Umi and Sora, two boys raised by dugongs who are strangely at home in the water. Scientists puzzle over the brothers' origins while ocean life vanishes around the world, and Ruka, pulled into their orbit, begins witnessing phenomena she cannot explain. Daisuke Igarashi ties the children's mystery to the wider question of what the sea itself is doing.