In 2014, a drastic attempt to combat climate change through stratospheric aerosol injection goes horribly wrong, plunging Earth into a new ice age that decimates much of its life. For seventeen long years, the few remaining humans live aboard a revolutionary self-sustaining train, the Snowpiercer, owned by the enigmatic transportation mogul, Wilford. Life aboard the train is a stark contrast; the affluent enjoy luxury in the front cars, while the destitute are relegated to filthy tail compartments, tightly monitored by armed guards.
Driven by his mentor Gilliam, Curtis Everett and his loyal ally Edgar Jamie Bell spearhead a rebellion among the tail passengers. To aid their fight, they liberate Namgoong Minsoo, a security expert held captive, who they lure with Kronole, a highly addictive drug derived from industrial waste. Namgoong insists that his daughter, Yona, also be freed. As they push forward, the group encounters heavily armed guards led by Minister Mason. In the heat of the chaos, Edgar is taken hostage, compelling Curtis to sacrifice him to capture Mason. He then forces her to command the remaining guards to lay down their arms while Edgar meets his tragic end.
The rebellious faction, now armed with Mason as a captive, ventures forward through the train's opulent cars. Namgoong and Yona identify an outside landmark and speculate on the possibility of thawing ice. As they arrive in a classroom, kids are indoctrinated with tales of Wilford's greatness. However, tragedy unfolds when a bald man infiltrates the tail army, shooting them with concealed firearms disguised as celebratory eggs for the train’s eighteenth global journey. The tide turns as Franco, one of Mason’s henchmen, unleashes a fury by executing Gilliam. In retaliation, Curtis executes Mason and they press ahead, but not without losing Grey and Tanya to Franco's ruthless attack.
In a pivotal moment before reaching the engine, Namgoong reveals his real intention behind collecting the Kronole—to use it as an explosive to aid in their escape from the train's confines, believing they can survive outside. Curtis confronts his haunting past, revealing to Namgoong that cannibalism was once the only means of survival in the tail, leading to a tragic history tied to the young Edgar. Curtis's desire for vengeance against Wilford for the horrific past leads him into a fateful encounter with Wilford’s assistant, Claude.
Faced with Wilford, Curtis is stunned to learn that his rising rebellion had been orchestrated, a cruel strategy to cut down the tail section's population to sustainable numbers, with Wilford subsequently suggesting Curtis take over the train. Just as it appears Curtis is about to accept this dark proposal, Yona interrupts, demanding a match. In a shocking twist, the floorboard reveals Tanya's son, Timmy, a child forced to work in the engine, alongside another child, Andrew's son, who bravely crawls into the engine core despite Curtis's pleas for safety.
In a race against time and danger, amidst a brutal showdown with Franco—the struggle culminates in Curtis handing Yona the match to ignite their explosive. As chaos erupts, the train meets its catastrophic end with an avalanche derailing it. Amidst the wreckage, though debris surrounds them, hope surfaces as Yona and Timmy emerge from the destruction to witness a majestic sight of a polar bear in the distance, signaling that life outside the train still exists.
