In a future where a failed climate-change experiment has killed all life except for the lucky few who boarded the Snowpiercer, a train that travels around the globe, a new class system emerges.
Set in 2031, the entire world is frozen except for those aboard the Snowpiercer. For seventeen years, the world's survivors are on a train hurtling around the globe creating their own economy and class system. Led by Curtis (Chris Evans), a group of lower-class citizens living in squalor at the back of the train are determined to get to the front of the train and spread the wealth around. Each section of the train holds new surprises for the group who have to battle their way through. A revolution is underway.
The nations release the substance CW-7 with the intention of controlling global warning, but accidentally it results in another Ice Age destroying Earth. The only survivors live in the train Snowpiercer which is driven by a perpetual motion engine created by the millionaire Wilford (Ed Harris) that lives in the engine part. In 2031, after seventeen years, there is a fight of classes inside the train since the oppressed lower class lives in dreadful conditions in the last wagons of the train while the elite lives in the front wagons. The lower class is fed with protein bars only; they are oppressed and punished by Minister Mason (Tilda Swinton) and her brutal guards; and their children are taken every now and then when they reach a certain height. The leaders Gilliam (Sir John Hurt), Curtis Everett (Chris Evans), and Edgar (Jamie Bell) are plotting a scheme to reach the engine and talk to Wilford about their conditions using the addicted security engineer Namgoong Minsoo (Song Kang-ho) and his clairvoyant daughter Yona (Ko Asung) that are imprisoned to open the locked doors. When the son of Tanya (Octavia Spencer), Tim (Marcanthonee Reis), is captured by Mason, they decide to execute their plan. There are many casualties and soon Curtis discovers that they have been manipulated by Wilford to balance the resources in the train, and why the magnate keeps the lower class alive.
It's 2031, seventeen years after authorities in seventy-nine countries decided collectively to combat global warming with the coolant CW-7. The result was the Earth being immediately sent into a deep freeze, killing all human life except a handful who now live on an "ark", a revolutionary train built before 2014 by a train aficionado named Wilford (Ed Harris), the train which at that time was considered useless and self-indulgent. The train is required to be constantly moving to generate energy for life inside, it which circumnavigates the globe on a single continuous track once every year. The train is still controlled in all of its aspects by Wilford, who has created three separate classes housed in different parts of the train: the privileged "one percent", who are at the front of the train, the workers who support both ends and occupy the middle of the train, and the "tail people", the masses who live at the back of the train like cattle in a windowless cattle car, they being solely fed protein bars, whose ingredients the tail people do not know. The face of Wilford's ideology to those in the tail end is Minister Mason (Tilda Swinton), as Wilford never ventures to that end of the train. Some of the policies which Wilford has implemented is taking many of the children from the tail end for an unknown purpose never to be seen by the people in the tail end again, and committing occasional indiscriminate genocide in the tail end, most of the tail people believing as a method of population control, and to demonstrate his dictatorial power in an effort to have a complacent populace amongst the masses. There have been many attempts of revolution by the tail people in the past, all quashed by Wilford. Curtis (Chris Evans), one of the tail people, wants to attempt another revolution, he believing all past attempts having failed because they have focused on Wilford, as opposed to what he feels needs to occur for power: take control of the engine at the front. Supporting Curtis in this fight are, amongst others: Gilliam (Sir John Hurt), the wise old man; Edgar (Jamie Bell), who Curtis took under his wing when he became orphaned in the early years; and Tanya (Octavia Spencer) and Andrew (Ewen Bremner), who want to find their children taken away. Along the way, they have to co-opt Namgoong Minsoo (Song Kang-ho), an addict of a synthetic hallucinogen called Kronole, he who designed the security system, namely the locking mechanism of the gates between all the train cars. As Curtis and his team try to work their way from car to car from the tail to the front, their priorities are influenced by what they see, what they learn, and by revelations including secrets from their individual pasts.
- In 2014, an attempt to counteract global warming through climate engineering backfires catastrophically, unintentionally causing an ice age that extinguishes all life except the inhabitants of the Snowpiercer, a massive train powered by a perpetual motion engine that travels a circumnavigational track, created by the transportation magnate Wilford. By 2031, elites inhabit the extravagant front cars and the "scum" inhabit the tail in squalid and brutal conditions. Under watch by Wilford's guards, they are brought only gelatinous protein bars to eat and kept in their place in the social order by Minister Mason, while sometimes small children are taken away.
Conspiring with his mentor Gilliam, Curtis Everett leads the tail passengers in a revolt that he plans will take them all the way up to the engine. Overpowering the guards, they release security expert Namgoong Minsu and his clairvoyant daughter Yona from the prison car so as to disable the locks between cars. They take the car where insects are ground up to make their protein bars, and Gilliam suggests that if they take the subsequent water supply car, they will control any negotiation with Wilford. Instead, they are ambushed by a mass of masked men with hatchets led by Franco the Elder under Mason's orders; after a bloody battle Curtis sacrifices his second-in-command Edgar to win the fight. Mason is taken captive and agrees to give free passage to Curtis, Namgoong, Yona, and three other rebels: Gilliam's bodyguard Grey, and Andrew and Tanya who have had their respective children Andy and Timmy taken away.
They travel through several luxurious cars and arrive at a classroom, where the teacher expounds to the children and the rebels on the greatness of Wilford and the "sacred engine". While distracted by the celebration of the New Year marking one circumnavigation of the globe, the teacher ambushes them, killing Andrew before Grey kills her. Further back, Franco and Mason's soldiers use the same distraction to kill the rebel army and many of the tail passengers. Franco executes Gilliam, and Curtis kills Mason in revenge. Curtis' group continues forward, followed by Franco, leading to a violent fight in a sauna car during which Franco kills Grey and Tanya before Curtis subdues him.
At the gate to the engine, Namgoong reveals that he plans to use Kronole, a hallucinogen he has gathered from the elites they passed, as an explosive to blow a hatch to the outside, as he observed signs the world outside is thawing. Curtis confesses to him that shortly after boarding the train, the tail passengers resorted to cannibalism to survive, and he is haunted by his part in it. He was nearly ready to kill infant Edgar when Gilliam offered his own arm instead. After years of disdain for Wilford, Curtis seeks to learn what Wilford's intentions were.
Wilford's assistant emerges from the engine, shoots Namgoong, and invites Curtis inside where he meets an aging Wilford. Wilford reveals to Curtis that his revolution was actually orchestrated by himself and Gilliam to reduce the population and maintain the balance of the sealed ecosystem, and subsequently orders the elimination of 74% of the remaining tail passengers. He explains the importance of using fear and chaos to maintain a necessary order and leadership on the train. After letting Curtis experience being alone for the first time in seventeen years, Wilford asks Curtis to replace him. Curtis appears ready to accept, when Yona runs in and pulls up a floorboard, showing Curtis that small children from the tail section are being trapped as replacement parts for "extinct" machinery; the tail section only serves to provide this resource to the engine. Curtis subdues Wilford and sacrifices an arm to save Timmy from this work.
Namgoong revives and as he is overrun by Franco and the front sectioners, he lights the fuse on the Kronole. Curtis and Namgoong tightly embrace Yona and Timmy, protecting them from the explosion. The noise triggers an avalanche that derails the train. Yona and Timmy, apparently the only survivors, emerge from the wreckage and see a polar bear, proof that life exists outside the train.