A cruise ship capsizes due to a severe storm. A human youngster who fell overboard is saved by Luchia, a small mermaid. She gave him a priceless gem that would save his life and heal him. Years later, Lucia needs the pearl once more to become a princess of the mermaids. She therefore mixes among the humans to try to locate the boy once more. She runs across Kaito, the young man she once helped, on the beach. Kaito, who also appears to "magically" draw evil, is now in grave peril because evil is also wanting her pearl and the pearl is still in his possession. Furthermore, despite not knowing that Lucia and the mermaid are the same person, Kaito still has feelings for the mermaid from that earlier period. She also finds it difficult to reveal who she really is because doing so might cause her to turn into sea foam. She must therefore wait for Kaito to discover for himself who the mermaid from that time is.
Gakuto, the bad guys' leader who resembles Kaito exactly, already has three mermaid princesses in his care. He needs all 7 mermaid princesses from the seven oceans in order to dominate the seas. In order to stop evil from contaminating the waters, Lucia and the other princesses must act immediately.
