I wanted to see this movie when it came out. Obviously, I didn't. Then I wanted to first watch the other vengeance films, Mr. Vengeance and Lady Vengeance, because all three are meant to be a kind of treatise. But, as I understand it, there's no particular order to the trilogy, so it doesn't really matter which film you watch first.
Dae-su is drunk. He spends a few hours at the police station before he's collected by Joo-hwan. They phone Dae-su's wife to let him know that he's coming home, but Dae-su disappears off the street and is never see or heard from again. He spends the next fifteen years locked in a room and then, quite suddenly, is set free. Dae-su is determined to get revenge on the one who imprisoned him, and ultimately, to find out why he was locked away.
Oldboy is a dark and violent film, full of tooth-pulling and tongue-cutting. But the mood is balanced out by humour and a kind of lightness. In spite of the heavy theme, the film doesn't take itself too seriously. Dae-su's vengeance quest is partly narrated by Dae-su himself and his sardonic wit contrasts the horror he inflicts on his enemies. He also meets Mi-do and her innocense further off sets the evil that surrounds Dae-su.
As I said, Oldboy is one of three movies that explores the theme of revenge. In this film, Dae-su is seeking revenge for acts perpetrated upon him. But when Dae-su finally confronts the man who shut him away, we learn Woo-jin's reasons for kidnapping Dae-su off the street. Both men, it seems, have a reason to hate the other, but who is more deserving of revenge? Dae-su's vengeance quest, however, is also a search for the truth: he was set free so that he may learn why he taken to begin with. For poor Dae-su there is no closure, for Woo-jin there's nothing else, and the movie is ultimately a comment on the emptiness of revenge.