Snowpiercer, the Great Ark Train, has kept the last remnants of humanity alive for almost seven years. A rigid class system maintains order, with First Class holding power over workers, while a condemned Prison Class struggles to survive in the Tail. Now, a grisly murder is stoking class division, so Melanie Cavill, the powerful head of hospitality, deputizes a dangerous rebel to help solve the killing - Andre Layton, the world's only surviving homicide detective.

Layton descends into Snowpiercer's black market with Till, searching for both the killer and a valuable commodity for his revolution. Melanie stages a prize fight to distract the passengers from mounting class tension.