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 uses his new position as train detective to investigate the murder while gathering intel for the revolution on the side. Meanwhile, Melanie faces a resource crisis, with potentially drastic consequences for the entire train.

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.

A twist in the murder investigation brings Layton face to face with the murderer, and dangerously close to Melanie’s secret.

Tensions between Third Class and First Class are boiling as Melanie stages the trial of the Snowpiercer killer. Third Class threatens a work stoppage, demanding representation on the jury, and Melanie makes a fateful decision about which side to favor.

Hiding out with help from his allies, Layton lays track for revolution. Meanwhile, an engineering emergency threatens every soul on Snowpiercer, and Melanie is the only one who can save them.