If you watched the drama, you'll probably be lulled by the actors' look.
The only person who was allegedly killed on the 4th floor was Moon-Jo.
As the policewoman herself remarks to her colleague: People dont care, because it is none of their business.
He comes across as a friendly and compassionate dentist; however, things are not what they seem.
The ending aswell, without any spoilers, leaves much upto viewer interpretation.
Still, it is extremely gloomy and difficult to watch since you feel sorry for the main character.