Lucia, Mermaid Princess of the North Pacific Ocean, sets out to land to find the boy she saved from a shipwreck seven years before the beginning of the story, to whom she had entrusted with her pink pearl. She eventually finds the boy: a surfer named Kaito Dōmoto. But as the human form Lucia assumed, named Lucia Nanami, bears very little resemblance to her mermaid form, Kaito cannot recognize her. She cannot tell Kaito who she really is; otherwise, according to mermaid folklore, she would turn into bubbles. She tries to convince Kaito into figuring out who she really is (since the legend does not say anything about the other person discovering a mermaid's true identity by himself).

At the same time, Lucia has been told that a group of water creatures or suiyō (水妖) have invaded the marine world and she must gather the 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, Mermaid Princesses who came up to dry land, use their pearls, turn into singing idols, and use their voices as an offensive power.
http://en.wikipedia.org/wiki/Mermaid_Melody_Pichi_Pichi_Pitch