In the 1970s in Hong Kong, Dou Fu, a second-year student at Primary School on Rooftop (天台小學) in the resettlement district, once offended the bandit Smelly Fei and had a grudge against him. On the day when the school gave out the grades, Dou Fu wanted to bring his progress grades home to let his mother know, but the grades were accidentally taken away. Dou Fu had to go to the complex Kowloon Walled City to find it. He met a new friend Zyun Lung and got help. The person finally found the score sheet, but at the same time, he encountered the hooligan Smelly Fei again and again, the enemy's road was narrow, and an inevitable confrontation, how could Dou Fu be safe and sound?