Bold (played by Cheng Kwun-On) leads a group of bandits to rob the South African diamond king. They find intermediary Li Yunfei (played by Chan Wai-Man) to sell the stolen goods, but during the transaction, they betray each other and shoot the buyer, allowing Li Yunfei to escape in the chaos. Undercover agent Zhang Lang (played by Stephen Chow) finds himself in a special situation at work, causing misunderstandings to grow deeper with his girlfriend Zhou Wenli (played by Joey Wong). Zhang Lang wants to quit, but under pressure from his old boss Officer Li (played by Lee Siu-Hung), he must approach Li Yunfei to investigate the diamond heist. At that time, Li Yunfei is being hunted by Bold's gang, and Zhang Lang helps him escape, earning Li Yunfei's trust. As they interact, Zhang Lang gradually discovers that Li Yunfei, as an old-school gangster, is loyal and righteous, leading them to become sworn brothers. Meanwhile, despite Zhang Lang's efforts, he cannot reveal his undercover identity to Li Wen, who is about to have a fake marriage to go to Canada. Zhang Lang's boss at the police station sees Li Yunfei merely as bait to lure Bold out, putting Zhang Lang in a difficult position between duty and loyalty.