KL Gangster Underworld begins after the incident of KL Gangster 1 (starring Aaron Aziz and Adi Putra), when dark syndicates in Kuala Lumpur lose their leaders due to being killed or imprisoned after being arrested by authorities. Each faction competes and kills one another to seize the empty throne left behind. Bloodshed, defections and sabotage run rampant throughout this period of chaos. In the midst of this turmoil, there is a syndicate member named Shah who wishes to retire and start a new life as an ordinary person, but is hindered by obstacles and impediments that force him to return to illegal activities. Unexpectedly, his return to the dark syndicate arena triggers a new revolution in the dynasty of dark syndicates in Kuala Lumpur.