杜决定以探虎穴的身份，卧底黑道之中，伺机报仇。Just as the wandering swordsman Du Mengfei (He Jiajin) was about to set off on a martial arts journey, he ran into Mu Wan’er (Guan Zhilin), a disciple of the Mu Longmen sect, who was being pursued by members of the criminal underworld. Du came to her rescue and Mu was extremely grateful. Shortly after, Du also saved a young girl named Li Sainan (Hui Yinghong) from being attacked by members of the underworld. However, Li misunderstood Du’s intentions and accused him of interfering with her affairs.Later, Du overheard a conversation about a grudge between two martial arts masters that occurred twenty years ago, and learned that the leader of the criminal underworld was the one who killed Li’s father. He also found out that Mu was the long-lost daughter of the leader of the underworld. Determined to seek revenge, Du decided to go undercover in the criminal underworld as a spy.