Bong Joon-ho uses the narrow, claustrophobic setting of the train to explore heavy themes:

Set in 2031, the film follows a failed attempt to stop global warming that triggered a catastrophic ice age, wiping out nearly all life on Earth. The only survivors reside on the , a massive, self-sustaining train that circumnavigates the globe once a year. The train is a rigid microcosm of society:

A major twist reveals that the revolutions themselves are often orchestrated as a means of population control.

Inhabited by the "scum" of the train, who live in squalid, overcrowded conditions and are fed "protein bars" made from ground insects.

A world of decadent luxury, occupied by the elite who enjoy fresh food, education, and lavish comforts.

The literal "front-to-back" hierarchy serves as a stark allegory for social inequality.