Chan Chi-kwong (Howard Sit) is a young boy who lives with his father (Felix Wong) and stepmother (Karen Mok).

He blames his stepmother for the suicide of his mother 3 years earlier and continually runs away from home.

One day, while walking to school, he meets an eccentric old man (Feng Xiaogang) in the park who accidentally pours a magic potion he has created down a drain.

Some of the potion enters a cut in his hand, and the next day he wakes up to find he has grown to the age of 20 overnight.

Kwong (Andy Lau) is thrilled that he has become an adult and is able to finally run away from home without being recognized.