There is a vacancy for the doorman position in your building and since you need the money and can't find a job you have no choice.
It's 1955 and for unknown reasons doppelgangers are more common than normal, so Doppelganger Detection Department (D.D.D.) has taken action on the matter.
Your job will be to allow or deny the entry of the subjects who request entry to the building, it seems easy, but be careful, you can't overlook any detail because you could be food for doppelgangers.
