"Gai Nie Men" tells the story of the seals that determine the rise and fall of the Gai Nie Men lineage in 1916, which are kept by three people, one of whom is the leader of the Green Gang, Ji Yuntian. One day, Ji Yuntian was assassinated, and some of the seals he kept disappeared. Police detective Zuo Nian went undercover to investigate the Gai Nie Men and was poisoned by the murderer. Zuo Nian's superior found his twin brother Zuo Si, hoping that he would continue to be an undercover agent in his place. Zuo Si, a commoner, did not have his brother's sense of mission and did not want to participate in this matter at all. After many twists and turns, Zuo Si finally decided to take the risk and continue the investigation, but he was involved in a shocking conspiracy that shocked Shanghai.