In a world where Aijin, beast-like creatures with exceptional powers, are oppressed by humans due to their scarcity, Rangetsu Ko and her twin brother, Sougetsu, struggle to survive. When Sougetsu is taken to serve as the prince's beast-servant and rumors of his demise surface, Rangetsu, consumed by rage, disguises herself as a man to join the military. Becoming Prince Tenyou's next beast-servant, Rangetsu anticipates revenge but finds unexpected complexities. As she delves deeper into the truth behind her brother's supposed death, Rangetsu is thrust into a perilous political web, questioning everything she once believed.