The story revolves around a mysterious place called "Angel's Island" that resides somewhere on the ocean seas. Living on Angel's Island are a species of merpeople that can live both in the sea and on land. They have the form of a human, but can breathe in water as well as on land. Luna is a princess of this species of mermaid living on Angel's Island, but is exiled for breaking some of their rules. As punishment, she is placed inside a sea shell, and set adrift across the sea.
While adrift, Luna is rescued from the sea shell when she is picked up by a passing ship. However, the people on the ship sell her into slavery, which causes Luna to lose her memory from the cruel treatment. Eiji Kusahara, the son of a wealthy Japanese family, finds Luna and rescues her from slavery; taking her into his home. There, she meets Eiji's younger sister, Akemi, who looks almost exactly like Luna. For some strange reason, Akemi is able to turn herself into a mermaid just like Luna on nothing more than a whim.
Before too long, mermaids from Angel's Island arrive and kidnap Eiji and Akemi, taking them back to the island. There, the sinister and deceitful Pyoma attempts to make Akemi a sacrificial offering, while Eiji meets Soleiu, Luna's older sister. Together, Eiji and Soleiu discover the mysteries behind Angel's Island.
- Luna: The mermaid princess who is exiled for breaking the rules of Angel's Island.
- Akemi Kusahara: Younger sister of Eiji Kusahara who happens to look exactly like Luna.
- Eiji Kusahara: Son of the wealthy Kusahara family who rescues Luna from a cruel fate as a slave.
- Soleiu: Luna's older sister.
- Pyoma: A wicked woman who intends for her own son, Pirene, to be the King of Angel's Island.
- Tuck/Kalpis as "Fuguhei":