The movie opens on a Joseon king facing a rebellion of his staff, defended by our main character Tae-yul (Jang Hyuk). He fights a duel against Min Seung-ho (Jung Man-sik), during which his sword shatters and damages his eyes. The king chooses to abdicate, releasing Tae-yul from his service. And at this point, we hit my biggest frustration in the movie as we jump forward about ten years to find Tae-yul living a quiet life in the woods with his daughter - who really wants to go into town. No resolution or explanation of the end of the rebellion ... but they let Tae-yul live? Why? We eventually find out they made this jump to keep the origins of his daughter a mystery so they could reveal it later.
I have no issue with action movies that have some depth to the plot, but I take issue when they try to add depth and instead add confusion. Maybe I would have understood it if I was Korean, as it's at least partially based on Joseon dynasty politics. Most of this became clear ... although partly because I did some Wikipedia reading on the plot of the movie. It also features a bunch of really, really nasty Chinese people (relatively accurate in that the Manchus were stomping all over Korea at the time) who are also slave traders, taking any Koreans they want at will.
Despite these problems, this is a fairly good action movie. The acting is good, as is the action, and the plot is interesting.