Sawa Yukimura runs a quiet bookshop in an alternate Meiji-era Tokyo where the Tokugawa shogunate still rules, but by night she serves as an executioner for the secret police force Nue. The last survivor of the Karasumori clan, she carries the clan's blue blood, which lets her transform into a beast-like Changeling in combat. She hunts Janome, the leader of the insurgent group Kuchinawa who slaughtered her family, while raising the orphaned girl Asahi as her own sister.