
Title: Old Boy

In 1988, a businessman named Oh Dae-su is arrested for drunkenness, missing his daughter's fourth birthday. After his friend Joo-hwan picks him up from the police station, they go to a phone booth for Dae-su to call home. While Joo-hwan is talking to Dae-su's wife, Dae-su is kidnapped and wakes up in a sealed hotel room, where food is delivered through a trap-door. Watching the television, Dae-su learns that his wife has been murdered and he's the prime suspect. Dae-su passes the time shadowboxing, planning revenge and attempting to dig a tunnel to escape.

2003, 15 years have passed since he was imprisoned. Just before digging himself to freedom, Dae-su is sedated, hypnotized and wakes up on a rooftop dressed in a suit. After telling everything that has happened so far to another man on the rooftop who later commits suicide, Dae-Su tests his fighting skills on a group of young thugs. Afterward, a mysterious beggar gives him money and a cell phone. Dae-su enters a sushi restaurant where he meets Mi-do, the restaurant's young chef. Dae-su receives a taunting phone call from his captor, who refuses to explain the reason for Dae-su's imprisonment. He collapses and is taken in by Mi-do. After he recovers, Dae-su tries to find his daughter and the location of his prison. Discovering that his daughter was adopted by a Swedish couple, he gives up trying to contact her. Dae-su locates the Chinese restaurant that made food for his prison and finds the prison by following a delivery man. It's a private prison where people can pay to have others incarcerated. Dae-su enters the prison and tortures the warden, Mr. Park, who doesn't know the identity of Dae-su's captor but reveals that Dae-su was imprisoned for "talking too much". While leaving the prison, Dae-su is attacked by a large number of guards and stabbed in the back with a knife but manages to defeat them all.

Dae-su's captor, a wealthy man named Lee Woo-jin, contacts Dae-su again and gives him an ultimatum: if Dae-su discovers the motive for his imprisonment within five days, Woo-jin will kill himself by stopping the pacemaker in his chest. Otherwise, he will kill Mi-do. As Dae-su and Mi-do become intimate, they have sex. Meanwhile, Joo-hwan tries to contact Dae-su with some important information about Woo-jin's sister but is murdered by Woo-jin, who was secretly following him after Dae-su removed all of Woo-jin's electronic bugs. Dae-su eventually recalls that he and Woo-jin had gone to the same high school, and he had witnessed Woo-jin committing incest with his own sister. Dae-su told Joo-hwan what he had seen before leaving the school, which leads to his classmates learning about the event. Rumors about Woo-jin's sister started to spread and she eventually committed suicide, leading a grief-stricken Woo-jin to seek revenge. Back in the present day, Woo-jin cuts off Mr. Park's hand, fulfilling an earlier threat by Dae-su, causing Mr. Park and his gang to seemingly join forces with Dae-su. Dae-su leaves Mi-do with Mr. Park and sets out to face Woo-jin.

At Woo-jin's penthouse, Woo-jin reveals that Mi-do is Dae-su's daughter. Woo-jin had orchestrated everything by using hypnosis to guide Dae-su to the sushi restaurant, arranging for them to meet and fall in love so that Dae-su will experience the same pain of incest that Woo-jin did. Dae-su attempts to attack Woo-jin but is beaten badly by Woo-jin's bodyguard. Using scissors as a weapon, Dae-su manages to stab the bodyguard in the ear, deafening him. The deafened bodyguard, now seriously hurt and enraged, attempts to kill Dae-su. Woo-jin, unable to call him off, intervenes and coldly shoots his bodyguard in the head with a Derringer pistol. Woo-jin reveals that Mr. Park is still working for him and threatens to tell the truth to Mi-do, who is being held in Mr. Park's new prison. Dae-su apologizes for his involvement in the death of Woo-jin's sister and humiliates himself by imitating a dog, begging Woo-jin not to tell Mi-do. When Woo-jin laughs unimpressed, Dae-su cuts out his own tongue as a sign of penance. Woo-jin finally accepts Dae-su's apology and tells Mr. Park to hide the truth from Mi-do. After entering the elevator, Woo-jin recalls his sister's suicide and shoots himself in the head with the Derringer.

In the epilogue, Dae-su finds the hypnotist from the prison to erase his knowledge of Mi-do being his daughter, so that they can stay happy together. While persuading to the hypnotist, Dae-Su repeats a sentence that he heard the man on the rooftop say, so she begins a hypnosis to erase his memories. Afterwards, Mi-do finds Dae-su lying in snow and confesses her love for him while the two embrace. There are no signs of the hypnotist, leaving it ambiguous if Dae-Su really even met her. Dae-su breaks into a wide smile, which is then slowly replaced by a look of pain. Whether the hypnosis really worked is left unknown.
