A grisly murder stokes class division on Snowpiercer, the Great Ark Train; Melanie Cavill, the powerful head of hospitality, deputises 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 intelligence for the revolution on the side; 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 shocking twist in the murder investigation brings Layton and Till's manhunt to a cat-and-mouse climax; Layton gets closer to Melanie's big secret, which may prove the most dangerous game of all.

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; Melanie makes a fateful decision about which side to favour.

Hiding out with help from his allies, Layton lays track for revolution; an engineering emergency threatens every soul on Snowpiercer.