It is a study of the depths of show business and its immoral inhabitants—the dark, shadowy world of a second-rate carnival filled with hustlers, scheming grifters, and Machiavellian femmes fatales.

Gresham attributed the origin of Nightmare Alley to conversations he had with a former carnival worker while they were both serving as volunteers with the Loyalist forces in the Spanish Civil War.

Clem explains that geeks are "made": a sideshow owner finds an alcoholic bum and offers him a temporary job with a steady supply of liquor.

Initially, the bum is only asked to pretend to be a geek, using a razor blade to slice chickens' necks and then faking the drinking of the blood.

He eventually leaves the carnival with the beautiful and naïve electric girl[1] Molly Cahill to perform a team code act.

Their act becomes very successful, but Stan grows bored and transforms himself into Reverend Carlisle, an upstanding spiritualist preacher offering séance sessions with the help of his medium.

A final reconnection with Zeena provides him an opportunity to again make living in the carnival, but he breaks completely and descends into alcoholism and depression when he discovers through a newspaper article that Lilith has wed Grindle.