The film begins by telling the story of an attempt to counteract global warming by engineering climate patterns with a chemical called CW7. The plan backfires and causes an Ice Age, which kills all life on earth except for the people who live on the Snowpiercer, a perpetual motion train that travels a span of track that loops one time around the globe in a year. It was created by a business magnate named Wilford, who runs the train.
It is now 2031, and the elites live in the front cars, where there is nearly endless luxury, while the "scum" inhabit the rear cars and live in horrid conditions where they are fed protein bars as food every day. Curtis, one of the citizens of the rear of the train, conspires to lead the passengers from the tail to revolt against the system and get all the way to the front of the train. The plan is kickstarted after two small children are taken from the rear of the train by armed guards and one of the fathers attempts to fight back, but is punished by being made to put his arm outside of the train in the extreme cold which leads to his arm being frozen off. The revolt begins.
They take the security and prison cars and awaken Namgoong Minsu, a security expert, and his daughter, Yona, who is clairvoyant. With Nam and Yona's help, the revolutionaries are able to disable the security locks between the cars. Curtis gives Nam and Yona a hallucinogenic drug called Kronole as payment for doing so. Gilliam, Curtis' mentor, suggests they take the water supply car in order to allow them leverage for negotiations with Wilford, but this leads to an ambush and a bloody battle where many lives are lost.
Curtis is able to take Mason, one of the head ministers of Wilford's train, hostage. They begin to travel through the nicer cars, where people live in decadent luxury. The group ends up in a school car where children parrot adulation for Wilford back to their teacher, an exceptionally cheerful pregnant woman. Not long into the class, the teacher pulls a gun on the revolutionaries and begins shooting, but Grey, one of the revolutionaries, promptly kills her. While this is happening, the rebel army in the rear of the train is killed and Gilliam is executed. Avenging Gilliam's death, Curtis kills Mason.
Nam and Curtis finally make it to the engine gate, where Nam plans to dynamite the door with his stock of Kronole. Before he can do so, Wilford's assistant shoots him, before inviting Curtis in to meet Wilford. Wilford reveals that he and Gilliam have been working together in order to keep the population down on the train. The revolution that Curtis began was actually orchestrated by Wilford to kill a certain number of passengers for population control. Wilford then asks Curtis to replace him. Curtis sees the children who were taken from the tail section being used as replacement parts underneath the floor in order to keep the train running, and declines Wilford's offer. Curtis then knocks out Wilford and sacrifices an arm to get the young child, Timmy, out of the machine.
Curtis finds Nam alive, and they protect the two remaining children as Nam and Yona set off the explosion of Kronole, which derails the train and causes an avalanche. After the wreckage, Yona and Timmy go outside the train, where they encounter a polar bear in the snowy landscape.
