OH Dae-su is an ordinary Seoul businessman with a wife and little daughter who, after a drunken night on the town, is locked up in a strange, private "prison" for 15 years. No one will tell him why he is there and whom his jailer is, but he is kept in reasonably comfortable quarters and has a TV to keep him company. While watching TV, he discovers that he has been framed for his wife's murder and realizes that, during one of the occasions in which he was knocked out with gas, someone has drawn blood from him and left it at the scene of the crime. The imprisonment lasts for fifteen years until Dae-su finds himself unexpectedly deposited on a grass-covered high-rise rooftop. OH Dae-su is determined to discover who his mysterious enemy is. He gets his first clue when a homeless man hands him a cell phone and a wallet full of cash. Later, while Dae-su is eating in a Japanese restaurant, the phone rings and a voice challenges him to find the reason for his imprisonment. Dae-su blacks out only to awaken in the apartment of the restaurant's pretty, young waitress, Mido. Mido helps him search for his hidden prison, but one night, Dae-su finds Mido exchanging emails with a mysterious stranger who seems to know all about him. Convinced Mido has betrayed him, he continues the search on his own. Dae-su locates his former prison and beats up the gangsters who served as his captors. A tape offers clues as to his enemy's motives, but not his identity. Dae-su blacks out on the street and after being helped into a cab by his mysterious, but still unnamed foe, he ends up back at Mido's. An old friend who owns a cyber café helps Dae-su discover that Mido's emailer, "Evergreen," is indeed the man who had him locked up. Dae-su is enraged by Mido's apparent betrayal, but a face-to-face confrontation with his smooth-talking adversary ends with Dae-Su's conviction that she is innocent. The man gives Dae-su five days to discover who he is and why he imprisoned him. He is told that if he succeeds, the man will kill himself; if he does not, he will kill Mido.