Lee Yong-gu is a mentally challenged man with an 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 on their own 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 who happens to find her 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, other inmates in the cell keep their guard against Yong-gu, then realize that there's something wrong 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 Yesung 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, Yesung 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

A father (Ryoo Seung-Ryong), who is mentally handicapped, but loves his daughter very much, is falsely accused of a crime and sent to prison. Later, his daughter Ye-Seung (Park Shin-Hye, who is a law school student, works to prove his innocence.

Back in 1997, Yong-Goo lives happily with his daughter Ye-Seung (Kal So-Won), even though he has the intelligence of a child. They stop by a shop everyday to look at a yellow Sailor Moon backpack that he hopes to buy his daughter. One day, they notice that the last yellow Sailor Moon backpack is being sold. Yong-Goo goes inside the store to plead his case, but the father buying the backpack ends up slapping Yong-Goo.

The following day, the girl who bought the yellow Sailor Moon backpack sees Yong-Goo working in the grocery store parking lot. She tells him that she knows another store that sells the same yellow backback. Yong-Goo follows the girl to a traditional outdoor market. A short while later, the girl is laying on the ground unconscious and Yong-Goo is trying to perform CPR. A woman walks by and calls the police.

Yong-Goo is accused of kidnap and murder. The father of the dead girl is also the chief of the National Police Agency. Yong-Goo is given the death penalty and incarcerated. In prison, Yong-Goo shares a jail cell with five other inmates. One day, Yong-Goo saves the life of his cellmate So Yang-Ho (Oh Dal-Su) from a rival prison gang leader. So Yang-Ho then offers to help Yong-Goo in any way he can to return the favor. Yong-Goo tells So Yang-Ho that he wants to see his daughter Ye-Seung. The five inmates then plan to make a miracle happen.