Some genre films are instantly recognizable because of their trademark plot, other films make it even easier and go for a title that tells you all you need to know. A Mob Story is a typical Hong Kong crime flick, only a tad darker and a smidgen more dramatic. That's Yau's limited influence on the film, but unless you are aware of his signature, I doubt anyone would actively notice.
Ah Chat is a seasoned contract killer. He started out when he was just seven years old and has been honing his skills ever since. Twenty years later he gets recognized on a mission, and he has to escape to Taiwan, where he knows a guy. There he bonds with a betel nut seller, but the booze and women drive a wedge between the friendship with his pal, and once again, Chat has to fear for his life.
This is another one of those films where Yau was able to gain experience and familiarize himself with the genre. The kills are a bit more brutal and some relations are a tad more developed, but it's really just a very basic genre film that's perfect for people who are looking for quality filler after completing the more accomplished films in this niche.