Snowpiercer, the great ark train, circles a frozen earth keeping humanity's last survivors alive - until a murder onboard threatens the established order.

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 shocking twist in the murder investigation brings Layton and Till's manhunt to a cat-and-mouse climax. But Layton is getting too close to Melanie's big secret, which may prove the most dangerous game of all.

Tensions between Third Class and First Class boil 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.

On a divided Snowpiercer, Melanie intensifies her search for Layton. Meanwhile, Layton is weaponizing her secret, and Third Class faces a reckoning when he presents them with a choice.

Revolution has finally come, and Layton leads the lower classes forward in armed rebellion. Melanie's house of cards collapses and she's in danger of becoming the first casualty in the battle for control of Snowpiercer.

In the revolution's aftermath, Layton struggles to govern the shell-shocked survivors. Melanie finally makes peace with her demons, only to realize the biggest threat to their survival is right over her shoulder.