Snowpiercer achieves remarkable narrative freedom precisely because director Bong Joon-ho believes in sticking to the rules. Other films that strive for thrill ride status, either in terms of vividly stirred intellect or full-throttled action (Snowpiercer is one of the rare beasts that goes for both), are all too quick to abandon internal logic when it serves the perceived need to set pulses pounding with clockwork regularity. Bong understands the value of setting parameters — maybe your own wonderfully gonzo parameters, but parameters nonetheless — and then honoring them. A movie doesn’t have to be believable to be plausible. It can achieve that necessary component to sound storytelling by locking into its own sharply defined world and never wavering, no matter the temptation. So Snowpiercer is set almost entirely on a ludicrously lengthy train that traverses the entire planet in the exact span of one year while serving as home to the remaining living population, trapped inside the vehicle by the hyper-arctic temperatures that resulted from a misguided attempt at countering the effects of global warming. The highly stratified class divisions of current society are compounded within the train with the different train cars serving as literal separations of the rich and poor, the downtrodden in the back and the wealthy up front. And it all makes perfect sense, both within its own carefully crafted structure and as allegorical commentary laced with viciously ingenious satire. Bong handles the slippery tone with skill, taking advantage of the journey forward on the train to unlock unexpected wonders with every new car broached by the revolutionary band of underclass heroes. He also builds images of unlikely beauty (Hong Kyung-pyo provides the aces cinematography) and expertly guides his cast through tricky performances that need to strike the same balance between gravitas and absurdity as the rest of the film. Chris Evans is sterling as the central hero, Alison Pill is a true ace in the hole as a crazy-eyed schoolteacher, and Tilda Swinton plays a villainous overseer with the sort of warped invention that no other actor should even attempt without a spotter.