Oldboy (2003) ending / spoiler
It is revealed that Woo-jin, Dae Su's captor, was in love with his sister. Oh Dae Su saw the pair together and spread rumours. Woo Jin's sister thought she was pregnant and thus killed herself with the help of her brother Woo-jin. This is the reason Oh Dae su was kidnapped. Woo-jin tells Oh Dae Su the reason why he was released after 15 years: it turns out Mido (the girl Dae Su met at the restaurant after his release) is actually Oh Dae Su's daughter and Woo-jin had raised her in secret. Woo-jin had both Mido and Oh Dae Su hypnotised to fall in love. Dae Su begs to Woo-jin to not tell Mido the truth and he cuts out part of his tongue. Woo-jin walks away and says "me and my sister were in love. I wish you luck" and then shoots himself. Oh Dae Su visits the hypnotist to erase the memories of what has happened. At first we are led to believe it works, but when Mido tells him she loves him, he smiles suggesting he still remembers. Mido never finds out that she is Oh Dae Su's daughter, Woo-jin allowed her not to find out.
Submitted by Rajesh Guru