You are a princess of a small country which belongs to the Yamato Commonwealth. Yamato is known to have hidden the ruins of an ancient civilization. It falls under control of a powerful country, Saint Cross, and you are almost forced to have an arranged marriage with its third prince Lorenzo! However, the true intention of Saint Cross is to acquire the “Gardiner’s Trove”, a weapon of the old ages hidden in Yamato, for military use. It is said that the casket of the Trove can be opened only by you. You and your personal bodyguard, Nagisa, run away from your castle in order to keep from handing over the Trove.
Who are you going to join hands with? Pirates? Or the navy...?

[from Official site]