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.

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

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

Revolution has finally come. As Layton leads the lower classes forward in armed rebellion, Melanie's house of cards collapses.

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.

In the revolution's aftermath, Layton and Melanie realize the greatest threat to their survival is right over their shoulder.

As the Big Alice crew are granted shore leave on Snowpiercer, Layton and Wilford have differing opinions on the future.

As the bomber is revealed, Layton comes to terms with his past decisions before making an irreversible call.

It all comes to a head as old adversaries clash, with New Eden hanging in the balance.

Milius and his Animal Squad storm the train. Melanie realizes something is wrong and shuts the vault doors to the engine. But when Ben's life is threatened, Melanie surrenders the engine.

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.

Layton hears the deal: Liana for Big Alice. He agrees and the trains dock at the silo. While Layton, Ben, Till, Ruth, and Alex stew in a holding room, Josie is taken to Headwood. Milius introduces his partner in crime: Wilford.

Nima needs more samples for his experiments, so Milius sends Wilford and the soldiers to collect. Privately, Milius tells Wolf to kill Wilford. Layton wakes up on an abandoned floor and is approached by humans with toxic gas scars.

Big Alice pulls into New Eden. Layton, Josie, and Liana sit stranded in the middle of nowhere, freezing to death, when a masked soldier appears. Can he rescue them? Alex finds herself in danger.

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?