During WWII Janusz(Jim Sturgess), young Polish officer held by Soviets as POW, is interrogated by NKVD (Stalinist secret security police). When Russians cannot force him to admit he is a spy, they bring his wife and extort from her a statement condemning Janusz. As a result he is sentenced to 20 years in one of the Gulag forced labour camps deep in Siberia.
At the camp , Janusz meets an American Mr Smith(Ed Harris), an actor Khabarov(Mark Strong), a hardened Russian criminal Valka(Colin Farrell), Polish artist Tomasz(Alexandru Potocean), a Latvian priest Voss(Gustaf Skarsgård), a Pole suffering from night blindness Kazik(Sebastian Urzendowsky), and an accountant from Yugoslavia Zoran(Dragos Bucur). Khabarov secretly tells Janusz that he is planing to escape south to Mongolia, passing Lake Baikal. It turns out that it's just Khabarov's fantasy in order to keep his own morale high, but Janusz decides to implement the plan. He runs away with Mr Smith, Valka, Voss, Tomasz, Zoran, and Kazik during a severe snowstorm in order to cover their tracks.
During a second night of their escape Kazik freezes to death after loosing his way to the fireplace while looking for wood and is later buried but the group. After many days of travel across the snows of Siberia they reach Lake Baikal. There they meet Polish girl Irena(Saoirse Ronan), who tells them a story of her parents being murdered by Russian soldiers, and her escape from a collective farm near Warsaw. Not all the details of her explanation how she ended up so far away from Europe check out (e.g. Warsaw is still under German occupation), but it turns out that she tried to conceal some more tragic experience. The group let's her join their quest.
When they reach an unpatrolled border between Russia and Mongolia Valka decides to stay, as he still sees Russia as his home country, and Josef Stalin as a hero. The rest continue to Ulaanbaatar, but soon they see images of Stalin and a red star. As it becomes clear that they will not be safe in Mongolia which is now also a Communist state, they realize that India is the closest refuge for them. As they continue south, across the Gobi desert, lack of water, sandstorms, skin burned to red-flesh, blisters and sun strokes weaken the group. Irena collapses several times and soon dies. The next to die is Tomasz. Smith seems on a verge of death to, but motivated by Janusz rejoins remaining Zoran and Voss and soon all four find a small stream of water and avoid dehydration.
While getting close to Himalayas they are guided to a Buddhist monastery, where they regain their strength. Smith decides to go to Lhasa from there to join American military mission fighting next to Chinese against Japan. The other three continue towards the Himalayas and they soon reach India.
At the end of the film Janusz finally reunites with his wife half a century later. For all those years he wanted to see his wife again so he could forgive her and she could thereby forgive herself.