Three murders have occurred in the Tokyo area. Left behind in all of the murders is a series of baffling numbers – a possible hint as to the next crime – that sets off an investigation to uncover the identity of a serial murderer. Kosuke Nitta of the Metropolitan Police Department Investigation Division deciphers the numbers and believes the location of the next offense will occur at the Hotel Cortesia in Tokyo. The police decide to investigate from within the Hotel Cortesia and Nitta is assigned to investigate posing as a clerk at its front desk. Selected to train Nitta in the ways of working at the hotel is their first-rate clerk Naomi Yamagishi. Nitta has focused himself on unveiling the “masks” of the hotel guests and to discover who the killer is.