In 1997, Li Longjiu (played by Liu Chenglong), a mentally retarded man with the IQ of only 6-year-old children, and his lovely daughter Yisheng (played by Ge Suyuan) depended on each other for life. Although their lives were simple and poor, they were full of happiness. One day, Longjiu, who was obsessed with buying a Sailor Moon schoolbag for his daughter, was accidentally involved in a case of abduction, rape and murder of a young child, and the deceased was the daughter of the police chief. Longjiu was ignorant and confused, and was thrown into prison in a daze. In cell No. 7, there are "social scum" with all the five poisons, such as smuggler Su Yanghao (played by Wu Dasu), fraudster Cui Chunhao (played by Park Yuanshang), adulterer Jiang Wanfan (played by Kim Jung-tae), intimidator Lao Xu (played by Kim Ki-chan) and robber Shen Fengzhi (played by Zheng Wanzhi). Longjiu's childlike pure heart gradually moved these "big bad guys", and they even took the risk to bring Yisheng into the cell to meet his father. In the dark and cold prison, cell No. 7 is full of sunshine...