Post-apocalyptic dystopias are a popular setting in the science fiction genre. With humanity so often on the brink of extinction, whether from a viral outbreak, alien invasion or any number of other catastrophes in our future, it can be tough to find new angles from which to approach the trope without getting the sense that it has been done before. Enter Korean auteur Bong Joon-ho, who (with the help of co-writer Kelly Masterson and a French graphic novel’s worth of source material) has taken the genre into new waters with Snowpiercer, an exciting take on the world after the end of the world. This Earth has been brought to death’s door thanks to an overzealous reaction to the global warming crisis (this is very much a post-An Inconvenient Truth story), as chemicals distributed into the atmosphere to cool the Earth has resulted in an unending and brutal new ice age that has wiped out all life on the planet. All life, that is, except the final gasp of humanity, confined to the train from which this film gets its title, a self-sustaining ecosystem that never stops running, circumnavigating the globe exactly once a year.

This train and its inhabitants would be the perfect sort of metaphor for society if it were not the only remaining representation of society itself (though it does have quite a bit to say about how things are done in the modern world). Our window into this wild future is Curtis (Chris Evans), a proud but broken man confined to the tail of the train where the outlook is bleakest and the amenities are few and far between. He has joined a cobbled-together ragtag group of revolutionaries led by Gilliam (John Hurt), who see salvation in the cushier parts of the train at its front that house the world’s remaining gentry. Pushing their way forward through the cars, they must contend with sadistic aristocrat Mason (Tilda Swinton) and a drug-addled super-locksmith (Song Kang-ho, a veteran of Bong Joon-ho films), as well as the increased treachery of a society that would much rather not be actively made aware of the existence of these untouchables.

It is a fascinating setting, and just the sort of approach to breathe life into a well-trodded genre. Knowing or expecting this going in could be a good way to explain why the first feels so staggeringly bland. Of course, the first act is the home of exposition and character work, which can sometimes be a slog in a high-concept story. Confined to the tail of the train, the setting is dark, drab and gritty, not all that unlike the future of Twelve Monkeys or Brazil for a new generation (it is no wonder Hurt’s character is named Gilliam). In part because of this familiar setting, Snowpiercer’s first act feels a little too generic. Once Tilda Swinton shows up, though, everything changes. Swinton is a slice of what is to come, a flamboyant explosion of character in the middle of a sea of grays and deep blues and sullen faces to match. She is a revelation, bringing the same sort of deep character work that is expected of her, fully disappearing into this bizarre Yorkshire accent and this Thatcher pea coat and these giant teeth she cannot help but caress with her tongue. Her speech in defense of the train’s caste system directed at the very people she is actively oppressing is a dizzying piece of rhetoric, and a glimpse into the depth of character hiding behind the locked gates of the train cars.

Indeed, when Curtis and crew break their way through to the front half of the train, Bong Joon-ho’s vision is brought into sharp relief. Colors bloom onto the screen as the corridors of cramped darkness are replaced by varied societal pods, the sort of amenities that would be expected of polite society before the world froze. A school, a spa, an aquarium, the front half of the train bleeds personality in the film’s second hour. It is a massive, almost infuriating cliche to say that the setting is a character in a film, but there is a legitimate case to be made that the Snowpiercer itself is the most important character of its movie. It is such a unique setting, with its corridor style influencing the inventive choreography of the action scenes and its ability to constantly shift settings without ever leaving the same place. For a film that ostensibly takes place in a series of horizontal tubes, the filmmakers imbue a wonderful visual diversity and sense of unpredictability to the proceedings.

This excitement is necessary, as there honestly is not all that much of note to the plot until the third act. Curtis is a cipher, the sort of good looking gritty guy who is expected to lead rebellions against dystopian societies, and Evans brings less to the role than he has in his other recent acting work. The revolutionaries are so nondescript that it can become almost difficult to root for them over their colorful, thoroughly engaging fascist oppressors who are full to bursting with creativity and personality. The plight of Curtis and his brothers in arms becomes secondary to just seeing what is behind the next gate, what new crazy sets and sequences lie just out of view.

This thirst for discovery, the elan with which the filmmakers approach the set design, pulls the narrative through to act three, when the film opens up and peddles its most intriguing moments. Moral dilemmas about at the head of the train. Allegiances and assumptions are tested, and the very nature of what it means to survive is called into question. It is here that Snowpiercer’s triumph is greatest, a crash course of multiple ideologies into one propulsive climax. The slow pacing of the opening act and much of the broad characterization melts away into the visceral and intellectual thrills of its excellent third act. It makes it all worthwhile.

The turnaround from the beginning of Snowpiercer to its end is somewhat remarkable. It comes out of the gate with such promise, appears to squander much of the good will of its premise in its first 45 minutes, only to deliver with flying colors in its late second and third acts. It is the sort of epic science fiction story that it should be, even if it takes its time getting there and does not have the most charismatic lead performance on which to lean. The pure cinematic joy of the last hour is more than what is needed to overcome its early shortcomings. Snowpiercer is a product of a genre that can easily get lost in its conventions, but establishes itself as proof that big idea science fiction remains alive and well.