In the underwater kingdom of Cerebrite, there live several mermaid princesses. One of them is Ninon, who was born in a peculiar manner unlike the other merfolk: she had the head of a fish, and the legs of a human. As such, the other merfolk shun her, and she feels out of place amongst even her own family.
Meanwhile, Ninon herself carries a desire to see the world beyond her home of the White Sea, a desire that is granted in a most unorthodox fashion on the night of her 21st birthday, in the form of a chance encounter with a nobleman named Vivien Bellerose.