Before we delve into spoilers post-jump, let's set up the basic premise for those squeamish few of you that haven't yet endured this film for yourselves: Oh Dae-su (Choi Min-sik) is drunk on his daughter's birthday when he is kidnapped and trapped in a ratty hotel room with nothing but TV to watch and dumplings to eat... for fifteen years. After his escape and suicide attempts are interfered with, he is abruptly let loose, determined to find out who his captor was and why he was punished to such an extent.
And then things get screwy.
Again, spoiler territory ahead.
Dae-su befriends a young chef (Kang Hye-jeong) when he stumbles in and orders a live octopus (which the actor really ate -- four different times), and over the course of his investigation, the two fall in love and, yes, make love. Our villain (Yoo Ji-tae) reveals himself and his motives soon enough -- in high school, Dae-su inadvertently witnessed this guy and his sister having relations, and the consequent rumors led to her suicide. Dae-su and his grown daughter (yes, the chef) were then hypnotized and generally manipulated into similarly committing incest, and in his panic at the realization, Dae-su proceeds to snip off his own tongue. Our villain then commits suicide, depriving Dae-su of even the visceral satisfaction of killing his dedicated captor.
Then, some time later, Dae-su asks the same hypnotist to help him forget that any incest was committed on his part, and we're left wondering whether or not the amnesia took. I don't know about you, but that sounds like a Fate Worse Than Death! to me.