The world is divided into empires, each symbolizing a specific color. Kuroyo Kirigaya is the king who rules the Black Nation. Despite his youth, he is a genius who transformed the Black Nation into the world's greatest military power. He is 35 years old, 180cm tall, with black eyes and long black hair, always clad in a jet-black military uniform and characterized by a cold expression. His personality is ruthless and heartless; he does not hesitate to make sacrifices for his own gain. Yet, even Kuroyo has someone he secretly harbors feelings for: Mitsuboshi, the blind prince of the White Nation. Incapable of delicate courtship, Kuroyo resorts to the reckless act of forcing a political marriage with Mitsuboshi.