Princess Sapphire is born with both a boy's heart and a girl's heart thanks to the mischief of Tink, a trainee angel, and because only a man may inherit the throne, she is raised as the kingdom's prince. The scheming Duke Duralumin, who stands to take the crown if her secret comes out, works to expose her, while Tink is sent down to take back the boy's heart. Sapphire keeps up her double life, rides out in disguise as the masked Princess Knight, and falls for Prince Franz Charming of the neighboring kingdom.