The year is 1931. Prince Tokugawa Yoshinobu holds absolute control over Japan. Remnants of the Meiji era’s culture can be seen around the city, but scientific technology and Japanese esoteric cosmology Onmyodo are also developing. Yet lurking behind the glitz is Kuchinawa, a dissident group planning the assassination of the prince. Tasked to extinguish these dissidents is Nue, the government’s secret executioner group. Sawa Yukimura, who works for this organization, suffered at the hands of the Kuchinawa boss. Her entire family was murdered and she dedicated her life to avenging their death.

Sawa Yukimura is orphaned after her family are murdered by Kuchinawa to obtain a quantity of their blue blood which has the ability to transform humans into "Changelings", demonic beasts associated with animals. She unwillingly becomes an assassin for Nue in the hope of seeking revenge on her family's murderers.