Lucia Nanami, the mermaid princess of the North Pacific Ocean, sets out on land to find the boy that she had saved from a tsunami wave seven years before the beginning of the story, to whom she fell in love and had entrusted her pink pearl. She eventually finds the boy: a teenage surfer named Kaito Dōmoto. However, Kaito does not recognize Lucia in her human form. She cannot directly tell Kaito who she really is; otherwise, according to mermaid legend, she would turn into bubbles and disappear. Lucia tries to convince Kaito into figuring out who she really is (since the legend doesn't say anything about the other person discovering a mermaid's true identity by him or her self).
At the same time, Lucia has been told that a group of water demons have invaded the sea world and she must gather six other Mermaid Princesses and their pearls to bring back the legendary goddess Aqua Regina to stop them. To fulfill this, she joins forces with Hanon and Rina, the Mermaid Princesses who came up to dry land, use their pearls, turn into singing Teen idols, and use their voices as an offensive power. After the water demons fail, the Black Beauty Sisters show up to steal the show. The Mermaid Princess believe in themselves and believe they can defeat the Black Beauty Sisters, which they do.