"KL Gangster" tells the tale of two brothers who get involved in the world of gangsterism. The older brother Malek (Aaron Aziz) was imprisoned for five years after being betrayed by his own gang which was started by Shark (Syamsul), the step brother of the most influential gangster in Kuala Lumpur, King (Ridzuan Hashim). Malek who leads a normal life after being released, is pulled back into the world he left behind after all those years. Especially since his younger brother Jai (Adyputra) is influenced by the gangsters and decides to work for Shark bringing everything down in chaos.

Two brothers, Malek and Jai, are indirectly thrust headlong into the world of gangsters and crime when their father died 10 years ago. Since their father's death, Malek, Jai and their mother have been threatened by gangsters whom their father owes money to. Desperate, the wild Jai turns to crime, and will do anything as long as it pays well. Jai's behaviour sees vehement opposition from Malek and their mother, which breaks up their family. To make things worse, Malek finds out that his sister Shera has been raped and their mother has cancer...