Okay, here are MASSIVE spoilers for Oldboy. You asked for it–brace yourself:
[spoiler] A Korean businessman, Dae-sue, is on his way home from work to his wife and young daughter. He is abducted and imprisoned in what looks like a hotel room. He doesn’t know who did this to him or why. He is regularly provided with food and water, and sometimes he is gassed to put him to sleep. After some time, he learns from the television in the room that his wife has been murdered and his daughter has been sent to live with foster parents. Dae-sun is named as the prime suspect in his wife’s murder.
After 15 years of this imprisonment in the same room, he is set free but still he does not know who imprisoned him or why. He is a dazed and broken man and wanders into a restaurant and talks with a young woman, Mi-do, who works there. She pities him and takes him back to her home so she can take care of him.
Within the next day or two, the man who had imprisoned Dae-sun reveals himself. Dae-sun does not seem to know the man (named Woo-jin). Woo-jin makes a proposition to Dae-sun. If Dae-sun can discover the reason that Woo-jin imprisoned him (within 3 days or something like that) then Woo-jin will kill himself. If Dae-sun does not figure the mystery out, then Woo-jin will kill the young woman, Mi-do, with whom Dae-sun has become very close.
As Dae-sun and Mi-do begin investigating to unravel Woo-jin’s motives, they fall in love despite their large age difference and have hot, passionate sex. Like, “I’ve been locked in a room alone for 15 years-” sex. He really fucks the heck out of her! 
Their investigation soon reveals that Dae-sun and Woo-jin had attended the same high school for a brief time. Dae-sun eventually remembers that one day in school he had spied on Woo-jin and one of the girls from the school having sex in an empty classroom. Dae-sun had made some comments to some other students about Woo-jin and the girl, and the rumors had spread. Dae-sun, however, had transferred to another school and never knew the whole story: The girl that Woo-jin had been screwing was actually his own sister! This sister, disgraced by the rumors of incest and the truth behind them, had killed herself.
Dae-sun now confronts Woo-jin with this information, having solved the mystery, and tells Woo-jin to live up to the deal and kill himself. Instead, Woo-jin gives Dae-sun a photo album. Dae-sun opens the album and sees a photo of his own wife and young daughter, as they were the last time he saw them before being imprisoned. Then as he flips through the subsequent photos, his wife no longer appears (she was murdered, remember?) and the girl is growing older and older in the pictures. As she matures in the photos over the years, she starts to look familiar. The final photos show her as a young woman and Dae-sun realizes with horror that the daughter he hasn’t seen in 15 years is the young woman Mi-do! The same woman that he has been having hot monkey sex with! :eek: Aaauuugghhh!
Woo-jin had orchestrated everything–Dae-sun’s imprisonment, the death of his wife, and his release 15 years later–in order to set Dae-sun up to commit incest with his daughter. (there is a somewhat far-fetched story involving hypnosis to explain how Dae-sun first found Mi-do in the restaurant and why she fell in love with him)
Dae-sun is insane with grief. He begs Woo-jin not to tell Mi-do about this. He also begs Woo-jin for forgiveness. As a sign of his grief, he cuts off his own tongue! Really!–he hacks through it with a pair of scissors! :eek: :eek:
Woo-jin savors for a moment the success of his ultimate revenge, then gets in the elevator from his penthouse, pulls out a gun, and blows his own brains out.
Now *that *is an evil, diabolical plan.[/spoiler]