Lee Yong-gu is a mentally challenged man with the intelligence of a 6-year-old, which is actually the age of his own daughter Ye-sung who is much smarter than her peers. The two of them lead a happy life while Yong-gu makes a living by working as a parking attendant at a local supermarket. But one day, when the police commissioner\’s young daughter dies in a strange accident, Yong-gu is the one who happens to find her. He is falsely accused and sentenced to death for abduction, sexual assault, and murder of a minor. Ye-sung is sent to a childcare institution and Yong-gu gets imprisoned and assigned to Cell No. 7, the harshest cell in a maximum security prison.
At first, the other inmates in the cell keep their guard against Yong-gu, then realize that there\’s something not quite right with him. Not understanding his own dire situation, Yong-gu only worries about his daughter. One day, he saves his fellow cellmate, an influential figure in the prison, from an attack by his rival. In return for saving his life, Yong-gu is granted a wish: to see his daughter Ye-sung again. The inmates put their heads together to carry out a daring operation and miraculously succeed in sneaking Ye-sung into Cell No. 7 during a religious event. While the father and daughter rejoice, the others realize it was relatively easy getting her in, but impossible getting her out. After a while, Ye-sung is detected and sent back, but during the time the inmates as well as the prison guards come to realize that Yong-gu was falsely charged. And they all put their heads together to prepare him for his final trial.