Dae-su Oh (Min-sik Choi) discovers that he was imprisoned because he discovered that a boy at school, Woo-jin Lee (Ji-tae Yu), was having an incestuous relationship with his sister. The sister commits suicide and Woo-jin seeks revenge. He waits for Dae-su to have a daughter, Mi-do (Hye-jeong Kang), then imprisons him for fifteen years. Mi-do grows up and Woo-jin hypnotises her and Dae-su into a sexual relationship. When Dae-su finds out what he's been doing, he cuts out his own tongue.
Thanks, Neil A!