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 Nitta Kosuke must go undercover and work again at the Hotel Cortesia Tokyo.
There, he meets Yamagishi Naomi, who works as a concierge. Without any information about the criminal, except that the person will be in attendance, Detective Nitta Kosuke works feverishly to find the criminal among the 500 attendees.