Shuang tian zhi zun (1973)

Two petty adolescent street urchins Lo and Lung become inseparable sworn brothers who survive the cruelty of the streets. They are separated after Lo gets caught by the law trying to steal food. Years later, Lung grows up to be an enforcer for a gambling syndicate not knowing that it also fronts as a narcotics ring. To contrast Lo is raised in a law enforcement household and is sent out to investigate drug activity in the region. They reunite under a fragile alliance on the opposite sides of the law as their interests spiral into collusion. Things turn deadly after drug boss Ma retaliates by having Lo's stepfather killed and framing Lung for the deed.