From MangaHelpers:
Hana has a mysterious tail installed directly into her spine that allows her to perform superhuman feats. This tail was installed into her by the professor known as Prof. Oyamada who raised Hana from birth. Ever since then, she grew up with her villagers in the small island and helped hunt for food for everyone using her tail.
One day, a mysterious boy washed ashore on her island. The boy claims to be the prince of Garlandra, the country where Prof. Oyamada hails from. He also claims that Hana's tail is actually his property and wants to take it home with him. Begrudgingly, Hana decides to follow Anthony. With Prince Anthony, Hana's battle-filled adventure begins.