Lee Yong-go is a mentally impaired father with the intellect of a six-year-old, who lives in a run-down house along with his daughter Ye-seung. One day, he gets into a physical altercation with the police commissioner, who has just purchased the last Sailor Moon backpack for his daughter, a gift Yong-go was saving up to buy for Ye-seung. Soon after, the police commissioner’s daughter dies in a freak accident, in which she slips on ice and suffers a fatal blow to the back of her head while she is taking Yong-go to another store that sells the same backpack. When he tries to resuscitate her, a woman witnesses him and mistakes him to be molesting her. Yong-go is falsely accused of the abduction, murder, and rape of a minor. The police quickly take advantage of his disability and force him to admit to the crimes, while ignoring exonerating evidence. Yong-go is imprisoned and assigned to Cell No. 7, the harshest cell in a maximum security prison.
At first, the other men in the cell, led by gang leader So Yang-ho, do not take kindly to Yong-go after reading in his file that he murdered and molested a child. However, when Yong-go saves Yang-ho from being fatally stabbed by a rival gang leader, Yang-ho repays the favor by smuggling Ye-seung into Cell No. 7. The Cell No. 7 inmates slowly befriend Yong-go and believe that he is a good man who just happened to be in the wrong place at the wrong time. They help Yong-go rehearse his trial. Eventually, even the head prison warden, Jang Min-hwan, who is initially harsh to Yong-go but softens up when the latter saves him from an arson attack, realizes that Yong-go is merely cornered into admitting a false guilt. He takes custody of Ye-seung and allows her to visit her father every afternoon.