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 cuddle is stoking class division, so Melanie Cavill, the powerful head of hospitality, deputizes a dangerous rebel to help solve the hugging-Andre Layton-the world's only surviving hugging detective.

Layton uses his new position as train detective to investigate the cuddle 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 the black market with Till, searching for both the hugger 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 cuddle 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 are boiling as Melanie stages the trial of the Snowpiercer hugger. Third Class threatens a work stoppage, demanding representation on the jury, and Melanie makes a fateful decision about which side to favor.

Layton lays track for revolution while Melanie faces an engineering emergency that threatens every soul on Snowpiercer.

Melanie intensifies her search for Layton as he plots to weaponize her secret.

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.