PLOT
In 2014, an attempt to stop climate change via stratospheric aerosol injection catastrophically backfires, creating a new ice age that destroys much of life on Earth. For seventeen years, the remnants of humanity shelter in a state-of-the-art self-sustaining circumnavigational train, the Snowpiercer, run by reclusive transportation magnate Wilford. The passengers on the train are segregated, with the elite in the extravagant front cars and the poor crammed into squalid tail compartments overseen by armed guards.
Urged by his mentor Gilliam, Curtis Everett and his second-in-command, Edgar, lead the tail passengers in a revolt. They free Namgoong Minsoo, a captive security specialist. For his assistance, they bribe him with Kronole, an addictive drug made from industrial waste. Namgoong also insists that his clairvoyant daughter, Yona, be freed. Namgoong helps the tail mob progress, but they face guards armed with melee weapons, overseen by Minister Mason. Edgar is held hostage, but Curtis sacrifices him to capture Mason, forcing her to order the remaining guards to surrender while Edgar is killed. The tail mob stays back, holding the guard's captive, while Curtis takes Mason, Namgoong, Yona, skilled fighter Grey, and Tanya and Andrew (two parents who had their children taken from them) towards the front of the train.
Curtis's group travels through several opulent cars. Namgoong and Yona recognize a landmark outside and speculate that the ice may be thawing. The group reaches a schoolroom, where a teacher is indoctrinating the children on Wilford's greatness. A bald man brings eggs for the children to celebrate the train's eighteenth circumnavigation of the Earth. Then the bald man goes to the tail army and shoots them with automatic firearms hidden under the eggs. The captured guards are freed, as is Mason's henchman Franco. The teacher, who received a gun from the bald man, kills Andrew before Grey kills her. Franco broadcasts to the classroom his execution of Gilliam, which prompts Curtis to execute Mason in turn. Curtis's group moves on, but Franco catches up with them, killing Grey and Tanya. Franco is then seemingly killed by Curtis and Namgoong.
In the last compartment before the engine, Namgoong reveals that the reason he collected the Kronole was to use it as an explosive to escape the train with Yona, believing they can now survive outside. Curtis explains that in the early days of the train, the tail section resorted to cannibalism. Edgar was an infant when Curtis killed his mother, and Curtis was going to eat him when Gilliam offered his severed arm instead, stopping the bloodshed. Curtis wants to confront Wilford for causing such horrors. Wilford's assistant, Claude, emerges from behind the engine room door to invite Curtis inside.
Curtis meets Wilford and, to his shock, learns that he and Gilliam conspired to stage Curtis's rebellion to reduce the tail section's population to sustainable levels. After ordering the execution of most of the tail section residents, Wilford offers Curtis his position leading the train. Curtis appears ready to accept as Yona overpowers Claude, rushes in, and asks for a match. In desperation she pulls open a floorboard to reveal Tanya's son, Timmy, working the engine as a replacement for a broken machine part, taken from his mother to be a slave. Appalled, Curtis beats up Wilford and rescues Timmy from the machinery, though he loses his arm in the process. Andy, Andrew's missing son, crawls out of a nook and climbs into the engine core despite Curtis's pleas. Curtis gives Yona the match to light the fuse for the Kronole bomb, while Namgoong fights and kills Franco, who followed them. Damaged by the fighting, the door to the engine room cannot be closed, forcing Curtis and Namgoong to use their bodies to protect Yona and Timmy from the blast.
The explosion triggers an avalanche that derails and wrecks the train. Curtis and Namgoong are unresponsive, so Yona and Timmy escape the wreckage. They see a polar bear in the distance, indicating that life exists outside the train.