The heroine, Ruka, who was banned from taking part in the activities of her club during the summer vacation, meets a strange boy named Umi. She is fascinated by Umi, who she sees swimming freely in an aquarium, and finds out that he, along with Sora, was raised by a dugong. At that time a meteorite fell into the sea and all the fish disappeared from the world. The mystery of the sea deepens and the story spreads to encompass the life of the earth.

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