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.
However she is found by Eji, a kind man which was reminded of his sister when he laid eyes upon her, rescuing her from the terrible fate of being a slave. After much insisting for repaying his kindness, Luna offered herself as his assistance and to try to be a friend to his sister. But unfortunate events take place as Eji's sister is confused for Luna, kidnapped by the very woman that wanted Luna dead.
Eji's journal to recover his sister will uncover secrets from Angel's hill, a big conspiracy and why Luna was sentence to death.