Specifically, well-meaning scientists accidentally unleash an ice age as they tried to curb the heating of the planet setting off an extinction event.
Luckily, mysterious billionaire Wilford builds a train with a perpetual-motion engine to circle the planet and keep only the most-deserving and wealthy of humans alive.
That translates into 3,000 survivors stuck on a 10-mile-long, 1,001-car train divided by classes — first (for high-priced, paid passengers); second and third classes (for those with specialized skill sets to keep the train running and passengers fed); and a final group who illegally hijacked some of the rear cars of the train.
Led by Andre Layton (Daveed Diggs) and disrespectfully called tailers, they are treated with disdain and nearly slave status by the other passengers. Of course, Layton and his rebel survivors want more and hatch a plot to take control of the Snowpiercer.