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Man on the Brink is a fictionalized dramatization of a true incident in the history of the triad gang in Hong Kong. A young police officer by the name of Ah Chui (Eddy Chan) goes undercover to obtain evidence on the gang, while another policeman, "Tai," is around to help him out. In order to make his cover pass muster with the gang, Ah Chui shares in their drinking, their wild parties, and their petty crimes. He steals and drives cars with so much expertise that he is used as the driver in a robbery, which goes well for the gang. Before long, Ah Chui is feeling the bad effects of his pretend life: he is stung by a mission that backfires for his friend Tai, and he chokes at not being able to tell his girlfriend the truth about what he is doing. Ah Chui slowly undergoes a radical transformation as he "walks on the brink" between stability and neurosis, and good and evil.