Taking place in a medieval European-like fairy tale setting, "Princess Knight" is the story of Sapphire who must pretend to be a male prince so she can inherit the throne of Silverland as women are not eligible to do so. As she is born, her father, the King, announces his baby is a boy instead of a girl. The reason for this is that the next-in-line to the throne, Duke Duralumin is an evil man who would repress the people if his son, Plastic, were to become king. Duralumin and his henchman Baron Nylon often scheme to take over the kingdom and attempts to prove that Sapphire is really a girl. Sapphire can keep the façade because, when she was born, she received the blue heart of a boy as well as the pink heart of a girl. Because of this, God sent Tink, a young angel-in-training, down to Earth to retrieve Sapphire's extra heart. Sapphire would not let Tink remove her boy's heart, however.