This is a Norwegian historical drama about how church tried to kill a small child. Okay, now a bit more serious. In medieval times Norway had its own civil struggle, which eventually resulted into country being split into two major parties - Birkebeinar and Balger. Each party tried hard to legitimize their position by getting king's support. Thing is, kings were often changed - some were killed of, some died... and it was not an easy task to determine the next rightful ruled due to totally fucked up inheritance laws. One of the notable moments of that struggle was when an infant heir to the throne - Haakon Haakonsson - was saved from death by Balger hands thanks to two Birkebeinar who carried him away from danger on skis, through the harsh Norwegian blizzard. Here, just look at this epic 19th century picture, representing those events:
Cool, right? Pretty much, vikings on skis. This event is actually so popular, that there are ski events named after it - like ski marathons, I think. And the movie is based on that trek. Obviously, since it all happened in winter of 1205/06, you can be sure that there's no way to tell how true any of it was. Then again, we can make some educated guessed. For example, movie uses quite a few historical names. Inge Bårdson, Inga of Varteig, Haakon Haakonsson himself... Hell, history knows the names of two guys who saved the future king - Torstein Skevla and Skjervald Skrukka, and those are the main heroes of the movie.
Details of the movie are, however, very much a fiction. As I said, there's no way to know what happened in particular, so it's obvious, that the movie-makers had to just make things up. And they did. For example, I haven't found the name of the main villain Gisle anywhere, so I would assume he is made up to make it all more dramatic. Story with the king's poisoning is half true - he did get ill, and died, and it was suspected, that he was poisoned by Margareta. On the other side of it, we can be sure that the story of Skjervald in overly dramatized on the movie - he wasn't accused as traitor, and he and Torstein were pretty much chosen for the simple fact that they were the best skiers. Then again, any of those guys could have had their family killed by the other side, so, you know, there might have been some truth in that part... Or not. Also, it's uncertain, how much fighting they had during those events. Certainly, they were fleeing from the Balger territory, and they were chased.
All in all, I liked the movie. It's a lose interpretation of historical events that were pretty important for Norway, and it's can be forgiven for not being too close to the truth for a simple reason of it all happening too long ago to have real truth known.
Also, seriously, knights on skis. It's epic.