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The plot is centered on a fish-girl who lives in an aquarium in her father's underwater castle with numerous smaller sisters. When her father takes her and her siblings on an outing in his four-flippered submarine, she is driven by a desire to see even more of the world and swims away. She ends up stranded on the shore of a small fishing town, and is rescued by a boy named Sōsuke, who cuts his finger in doing so. She licks his wound when he picks her up, which heals it in short time. After taking a great liking to her, Sōsuke names her Ponyo and promises to protect her forever.