Lee Yong Gu is a mentally impaired father, who lives in a run-down house along with his daughter of the same age, 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 while she is taking Yong Gu 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 Gu 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 committing the crimes, while ignoring exonerating evidence. Yong Gu is imprisoned and assigned to Cell No. 7, the harshest cell in a maximum security prison.
(Source: Drama Wiki Fandom)