*WARNING: SPOILER ALLERT*
Oldboy begins in 1988 when the main character, Oh Dae-su (Choi Min-sik), is mysteriously kidnapped on the night of his daughter’s birthday. For fifteen years, Oh Dae-su is confined to a hotel room without reason. His only window to the outside world is a television set. Through the television he learns that is wife has been murdered. In his strange absence he becomes the main suspect. Each day Dae-su practices boxing and attempts escape.
One day, Dae-su is released on the roof of an apartment building. From a small trunk, he emerges into the world. Unable to figure out why he was kidnapped or who kidnapped him, he goes to a sushi restaurant where he meets Mi-do (Kang Hye-jung). Oh Dae-su faints at the restaurant and Mi-do takes him back to her apartment. Upon waking, Dae-su briefly loses self-control and tries to attack Mi-do. However, the two reconcile and become friends. Mi-do agrees to help Dae-su search for his daughter.
They learn that Dae-su’s daughter was adopted by a Swedish family after his wife was murdered. Distraught by the news, Dae-su continues to search for his kidnapper. He tracks down the restaurant that made the dumplings he ate each day in captivity. From there he finds the delivery boy and then the make-shift prison. Returning to the private prison to get information, Dae-su must later fight his way out of the building. The famous fight scene involves Dae-su armed with a hammer against multiple henchmen.
Eventually, Dae-su and his captor meet. His captor Lee Woo-jin (Yoo Ji-tae) gives Dae-su five days to solve the mystery of his imprisonment. If he solves the mystery, Woo-jin will kill himself, if Dae-su cannot solve the mystery, Woo-jin will kill Mi-do. Dae-su and Mi-do work together to understand Dae-su’s imprisonment and in the process fall in love. In a moment of intense passion the two have sex. Dae-su begins to put together the pieces of his connection to Woo-jin, realizing that they both attended the same high school. As a teenager, Dae-su witnessed Woo-jin and his sister’s incestuous relationship. Dae-su then accidentally spreads the rumor of their relationship and Soo-ah (Woo-jin’s sister) commits suicide.
Solving the mystery, Dae-su rushes to Woo-jin’s penthouse apartment. There, Woo-jin reveals that Mi-do is actually Dae-su’s daughter and that he used hypnosis to orchestrate their meeting and attraction. In a moment of extreme anguish, Dae-su cuts out his own tongue, Woo-jin commits suicide, and the penthouse fills with blood.
In a flash-forward, Dae-su is hypnotized again to forget the terrible act he has committed. Dae-su and Mi-do meet again, but the success of the hypnosis is unclear.
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