In Shanghai during the 1930s to 1940s, gangs ran rampant, and drug trafficking was widespread. Drug lord Brother Chen held immense power, making it difficult for the police to gather evidence. Ren and Mei unexpectedly reunited during one transaction, but it was part of the intricate schemes of Brother Gui, Brother Chen, and the police. Amid suspicion and trials, Ren, an undercover agent for the police, infiltrated the criminal underworld and successfully helped the authorities apprehend Brother Chen and an international drug ring.