In the fairy-tale kingdom of Silverland, a rigid succession law demands that only a male heir can inherit the throne. To prevent the wicked Duke Duralumin and his inept son, Prince Plastic, from seizing power, a courageous princess named Sapphire is raised from birth as a boy. She was born with a unique condition: two hearts, one a brave blue heart and the other a gentle pink heart, granted by a celestial intervention before her birth. While navigating her public duty as the kingdom’s prince, Sapphire secretly lives the life of a princess and fights injustice under the masked persona of the Phantom Knight, also known as the Ribbon Knight.

The central conflict ignites as Duke Duralumin, aided by the nefarious Lord Nylon, grows suspicious of the ruse and schemes to expose Sapphire’s true gender. As enemies close in and political pressures mount, Sapphire is forced to confront the harsh destiny laid out before her. The story follows her journey as a lone hero who refuses to be bound by the identity others expect of her. She fights to protect her family, reclaim her rightful place as ruler, and challenge the oppressive laws of her kingdom while grappling with her own dual nature.

Inspired by the Takarazuka Revue, an all-female musical theater troupe, the narrative explores themes of identity, freedom, and defiance against fate. The protagonist’s determination to forge her own path drives the action, culminating in a battle against both physical adversaries and the societal norms of her world. The film adapts the classic 1953 manga Princess Knight by Osamu Tezuka.