Rangetsu is an Ajin, marked by beast traits and born into a kingdom that forces her kind into service: the men into the military, the women into the brothels. When her twin brother Sougetsu is taken to serve Prince Tenyou as a beast-servant and later reported dead, Rangetsu disguises herself as a man to enlist and get close enough to the palace to learn the truth. She ends up becoming Tenyou's own beast-servant, only to find the prince is not the villain his reputation promised, leaving her to decide what avenging her brother is actually worth.