Princess Sapphire is a girl raised as a Prince. Through the mischief of an angel, the princess is born with both a girl's and a boy's heart. Since there is no boy successor in her kingdom, Sapphire is raised as a boy, but evil ministers try to reveal her secret. Unable to put up with the kind of vicious conduct prevailing in the kingdom, Sapphire disguises herself as "Princess Knight" and wields her sword of justice.