Lucia Nanami, mermaid princess of the North Pacific, comes ashore in human form to find the surfer boy, Kaito Domoto, she rescued from a tsunami seven years ago and entrusted with her pink pearl. He no longer recognizes her, and she cannot reveal what she is. As water demons begin attacking the seas, Lucia and fellow princesses Hanon and Rina use their pearls to transform into idol singers whose voices are their only weapon, gathering the seven mermaid princesses' pearls to summon the goddess Aqua Regina.