Lee Yong-gu is a mentally impaired man with the intellect of a six-year-old. He 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-gu 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 taking Yong-gu to another store that sells the same backpack. When Yong-gu tries to resuscitate her, a bystander mistakenly thinks he is molesting her. Yong-gu is falsely accused of the abduction, murder and rape of a minor. Police quickly take advantage of his disability and force him to confess to the crimes while ignoring exonerating evidence. Yong-gu is imprisoned and assigned to Cell No. 7, the harshest cell in a maximum security prison.