Three murder cases take place in Tokyo. Numbers are left at the murder scenes, possibly hinting at where the next murder will take place. An investigation begins of the serial killer and Detective Kosuke Nitta works on the case. He believes the next murder will take place at Hotel Koruteshia Tokyo. To catch the killer, Kosuke Nitta goes undercover and begins working at the front desk of the hotel. Naomi Yamagishi works at the front desk of Hotel Koruteshia Tokyo. She is selected to train Kosuke Nitta for his front desk job, but they can't get along. Kosuke Nitta is focused on catching the killer, but Naomi Yamagishi prioritizes the safety of the guests. As they begin to understand each others differences, they also begin to build a trust between each other. Who is the murderer?

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.