Security 51 is a grim security checkpoint simulator where the key task is filtering personnel within a top-secret underground complex. The plot takes you into the depths of Area 51, which conceals laboratories for the study of anomalies and extraterrestrial entities in kilometers-long shafts. Taking up a post in front of the cargo elevator, the player verifies identities and checks access permissions. Gradually, routine bureaucracy becomes permeated with paranoia, as even perfect documents can conceal the presence of a foreign organism. A misidentification threatens a fatal breakthrough of uncontrolled threats to the surface. Security guards must decide people's fates every shift, relying on strict instructions and intuition.