Ajin boys who show signs of special abilities are conscripted to serve in the imperial palace as beast-servants--status symbols and shields for their royal masters, to be kept or discarded on a whim. When they were children, Rangetsu's twin brother Sogetsu was ripped from her arms and sent to the palace to attend Prince Tenyou as a beast-servant, where he quickly fell victim to bloody dynastic intrigues. Now in a world that promises only bitterness, Rangetsu's one hope at avenging her brother is to disguise herself as a man and find a way into the palace!

Female Ajin are prohibited from serving in the palace, and so Rangetsu's revenge depends on her ability to maintain her ruse. But when the attendant Taihaku discovers the truth and Third Prince Kougai's suspicious are roused, will Rangetsu lose her place by Prince Tenyou's side as well as her chance at vengeance?