According to Wikipedia, the movie is set during the Second Manchu invasion of Korea (~1640). Our main character is Nam-yi (Park Hae-il): at the beginning of the movie, he, his very young sister, and their father are hunted as traitors. Wikipedia explains that this is because their father was a supporter of the previous deposed king. Their father dies to save them, and they grow up hidden by Mu-seon, a friend of their father's (who is, like their father, an excellent archer). The next big event of the movie is the wedding of Mu-seon's son Seo-goon (Kim Mu-yeol) to Nam-yi's sister (Moon Chae-won), but this is interrupted by a horde of evil Manchus slaughtering and collecting slaves. Nam-yi is out hunting, so he's not in the village at the time - but he sees what happens.
That's perhaps the first half of the movie, and at that point the plot pretty much terminates because now they're set up to have a running archery battle for the rest of the movie as the outnumbered Nam-yi takes on an elite squad of Manchu archers who try to track him through the woods as he picks them off one by one (or two).
It's violent and fairly well done (although I could have done without the occasional fits of un-steady cam), but like most revenge flicks it ends badly for pretty much everyone. Not bad, but strikes me as being for a rather specialized audience even among fans of action movies. I also suspect that the archery isn't accurately portrayed: remember "Wanted" and the amazing bullshit talent of being able to "curve bullets?" It's like that except for arrows.