A grisly murder stokes class division on Snowpiercer, the Great Ark Train. 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 the 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.

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.

The rebel forces are on their heels when a dangerous foe joins them. A plan is hatched to take the train, but it may destroy them all.

Wilford presses forward on his endgame while those who are close with Layton grow anxious to learn their fates; Josie tests her newfound ability.

It's a game of cat and mouse as both Layton and Wilford vie for the upper hand.

Big Alice catches up to Snowpiercer. Milius radios them, inviting Layton over. Sensing a trap, Layton and Josie sneak onto Snowpiercer. Nima is worried Layton will be gassed and attempts to disarm the Greenhouse Car.

Nima heads for Snowpiercer. Layton and the Tailies fight their way uptrain when Ruth is captured. Nima tries to cut a deal and runs to the rocket control room to initiate the launch sequence, but who is trying to sabotage the rocket?