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 in both the sea and on land. They have the form of a human, but can breath 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. Meanwhile Eiji Kusahara, the son of a wealthy Japanese family, meets Soleiu (Luna's older sister) and they try to discover the mysteries behind Angel's Island.

Now available in a fancy new omnibus, both volumes in one: https://www.amazon.co.jp/dp/B014VZ3I5S/

Volumes 75 and 76 of the Osamu Tezuka Complete Works