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. But one day, when the police commissioner’s young daughter dies in a accident, Yong-gu who happens to find her is falsely accused and gets imprisoned and assigned to Cell No.7. One day, he saves his fellow cellmate from an attack by his rival. In return for saving his life, Yong-gu is granted a wish to see his daughter again. The inmates put their heads together to carry out a daring operation and succeed in sneaking Ye-sung into Cell No.7.