It's not hard to see why this is being picked up by American studios, since it concerns America's favorite topic: revenge. The story follows Oh Daesu (or Dae-su Oh if you want to be all Eastern about it) as he is kidnapped and locked in a reasonably comfortable hotel room. His only contact with the outside world is a small slot in the door through which his meals are passed, and the paltry 266 channels of cable that he is allowed on his Premium Gold package.
For 15 years, he sits in his cell, going slightly mad, training himself to be a superfighter, and chipping away at the outside wall with a chopstick. Just as he is about to remove the final brick that will give him access to the world of freedom, he is drugged, dressed in fine clothes and left on a rooftop. Then, of course, he must try to figure who kidnapped him and why, and, in the process, he has to physically harm fellow human beings through the use of violence.
The first hour of this film is pretty great. Sadly, this is a two-hour long film, which means it's roughly one hour too long. The problems really start to mount in the second half, where it becomes clear that the clever setup is not matched by a clever resolution. Basically, given the premise of the mysteriously imprisoned man, pretty much anybody could have produced a better reason for his imprisonment than the one given. In fact, you might want to go to the first half of the movie, then just sit in the theater parking lot with your eyes closed and try to imagine how you'd like it to end.
It's hard to describe what's wrong with the resolution, since this is where the mystery is revealed, and it would be wrong of me to mention that Rosebud is his sled, there's one in the escape pod with her, his wife's head is in the box, he really is the Russian spy, he was dressing up like his mother and killing people, they were saying "he'd kill us if he had the chance," he and Tyler are the same person, he goes to her wedding and takes her away and then they look uncomfortable, he lives one perfect day and then time goes back to normal, it climbs up the Empire State Building and then falls to its death, and it was just a dream. Nonetheless, I can say that there is nothing in the first half that sets up the second half, and in mysteries like this, it's considered a courtesy to the audience to give some hint about what's happening.
Instead, it comes across as random and out-of-the-blue. That could be OK, but it also isn't a very satisfying answer. It may be that this plot point relies too much on cultural constructs that are specific to Korean culture, or at least not terribly cognate with American culture.
Still, there are some great moments in this film. The hotel room where Oh Daesu is imprisoned is perfectly ordinary and yet incredibly trippy. The whole idea of being locked in a mid-priced hotel room, with private bath and cable, could be a gigantic metaphor for modern living. It doesn't really play out that way in the end, but it's at least suggested in the beginning.
And the initial build-up of the mystery is handled expertly. In fact, it's kind of a shock when the film starts to falter, because it completely loses the impetus it had and turns rapidly from a fast-paced thriller to a brutally slow slog-through psychological history. Director Chan-wook Park (probably best known for Sympathy for Mr. Vengeance and the intriguingly titled Sympathy for Lady Vengeance) clearly has basic directorial chops, and his visual sensibility is dead-on, but with Oldboy, he wasn't able to finish what he started.
To his credit, he manages to produce imagery that's both intriguing in its own right, and at the same time never fails to be informative. A lot of modern directors switch between the two, alternating the pretty shots with the ones that advance the story, but the old-school masters liked the economy of placing plot foremost, and adding flash only when it could be worked in to the larger goal of storytelling.
It's too bad Park couldn't have maintained this economical sensibility when it came to the narrative elements of the film. I imagine when they remake it for American audiences, the solution to the mystery will be quite different, so it might be fun to see this and then the American version for the purpose of writing that eighth-grade "compare and contrast" paper you've been putting off for the last 20 years. Otherwise, though, I'd recommend giving it a pass.