Rei, biologically female, possesses both a blue heart symbolizing male traits and a pink heart representing female traits, a result of the angel Tink's pre-birth mistake. To comply with Silverland Kingdom's law forbidding female succession, Rei is publicly raised as the male Prince Sapphire. Her parents and a trusted maid keep her biological sex a secret, known only to a select few.

Adopting the secret identity of the Ribbon Knight, Rei battles threats to Silverland and upholds justice while maintaining the prince persona. This double life demands swift transitions between courtly duties and heroic action. Duke Duralumin actively opposes Rei, seeking to expose her biological sex to disrupt the royal succession and further his own ambitions, driving conflict through court intrigue and external dangers.

Prince Franz Charming interacts with Rei, becoming romantically interested upon discovering her true biological identity. The resolution involves formal acknowledgment of Rei's female identity, leading to an inheritance solution that ensures Silverland's stability while reconciling her biological reality with the throne's demands.