Born with both a boy's heart and a girl's heart, Sapphire is raised as a prince in the kingdom of Silverland so she can inherit the throne. Women aren't allowed to rule there, and her father's announcement that his newborn is a son is meant to keep the crown from falling to the scheming Duke Duralumin. Sapphire grows up performing daring feats as Silverland's crown prince by day, guarding the secret of what she really is at every turn. Duralumin, suspecting the truth, schemes to expose her and seize the throne for himself.