In the kingdom of Silverland, a celestial blunder sets two children on a collision course with fate. The angel Tink, entrusted with assigning souls to newborns, makes a critical mistake: the princess Sapphire is born with both a male and a female soul, while the son of Duke Duralumin, Plastic, comes into the world without any soul at all. To prevent the ambitious and ruthless Duke from seizing the throne, the king announces that Sapphire is a boy, raising her as Prince Sapphire. Meanwhile, Plastic grows up as a hollow shell, unaware of what he lacks.

Years later, the witch Madame Hell reveals to Duke Duralumin that his son’s missing soul resides inside Sapphire. The duke schemes to reclaim it, setting in motion a chain of events that entangles Sapphire, Plastic, and Prince Franz of neighboring Goldland. Franz, who has befriended Plastic, becomes enchanted by both the princely Sapphire and the mysterious masked knight who appears at night. As dark forces gather, Plastic learns the truth about his own existence and must decide what he is willing to sacrifice to become whole. The story follows the intertwined journeys of these three—Sapphire struggling to maintain her dual identity, Plastic seeking his lost soul, and Franz caught between loyalty and love—all while the malevolent Duke Duralumin and the witch Madame Hell pull the strings from the shadows. The narrative weaves together themes of identity, belonging, and the price of living with an incomplete heart, set against a medieval fantasy backdrop of castles, magic, and sword fights. The manga, though left unfinished, explores the tension between the two souls and the desperate choices each character makes as their destinies converge.