An anonymous letter is delivered to the Tokyo Metropolitan Police Department. The letter states that a criminal of a murder case will appear at the countdown party Masquerade Night that will be held at Hotel Cortesia Tokyo. Detective Kosuke Nitta must go undercover and work again at the Hotel Cortesia Tokyo. There, he meets Naomi Yamagishi, who works as a concierge. Without any information about the criminal, except that the person will be in attendance, Detective Kosuke Nitta works feverishly to find the criminal among the 500 attendees.
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.