Princess Shirahoshi, famous in the world as the Mermaid Princess, is a giant mermaid (12 meters tall) and the princess of the Ryugu Kingdom, as the youngest of King Neptune's children. Having been locked up and raised inside an impregnable tower for most of her life thanks to the constant threat of Vander Decken IX, Shirahoshi wishes to know more about the outside world, following the will of her late mother, Queen Otohime.