The series is set in China during the Ming dynasty. Zhu Wushi, the uncle of the reigning Emperor, has been ordered by the previous emperor to establish a secret service called "Dragon Defending Mountain Manor" to protect the monarchy. Zhu, who is nicknamed "Tiedan Shenhou" has four elite secret agents serving under him. They are known by their codenames: "Heaven Number One", "Earth Number One", "Mysterious Number One", and "Yellow Number One". The Dragon Defending Mountain Manor has an ongoing rivalry with the Eastern Depot, another government spy agency led by the eunuch Cao Zhengchun. The Emperor knows that Zhu Wushi and Cao Zhengchun are equally influential in court politics and has been trying to use both of them to balance against each other. The four agents have to assist Zhu Wushi in curbing Cao Zhengchun's power, preserve the secret service's position in the imperial court, and defend the Emperor from internal and external threats.
