Lucia, a mermaid princess from the North Pacific, comes to the sea to look for the boy she saved seven years ago. The boy was drowning and she saved his life with her pink pearl and gave him her most precious pink pearl. Seven years later, she comes to the land to find the boy to get the pearl back for inheritance. She finds the boy, Kaito Domoto, but Lucia, who has transformed into a human, Nanami Lucia, is not similar to her mermaid form; she cannot tell Kaito her true identity, otherwise she will turn into foam. So she tries to make Kaito figure out her true identity. Meanwhile, Lucia learns that a group of water fairies have invaded the sea world. She must gather seven mermaid princesses and their pearls to revive the legendary goddess of the sea, Arca Regina, and stop the water fairies' plot. To do this, she teamed up with six other mermaid princesses who came to the land, Boeing, Lina, Xianglian, Noelle, Coco and Sara, and used their pearls to transform into singing idols, using their singing as weapons to attack the water monsters.