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!

On the eve of war, a letter from a potential ally arrives at the imperial palace, foretelling Prince Kougai's coming death in battle. The letter is persuasive enough that Prince Tenyou and the emperor begin to make plans to circumvent their brother's dark fate. But according to the letter, the only way for Prince Kougai to live is for someone else to die in his place!