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. 永远的海棠。卿本佳人。成是非&云罗郡主。天地玄黄天下第一。最后的武侠。有我最爱的海棠，一刀。一部不错的武侠剧。那时候的电视剧真有意思。剧美人美音乐美。那时候觉得归海一刀好帅啊。 段天涯（李亚鹏 饰），归海一刀（霍建华 饰），上官海棠（叶璇 饰），成是非（郭晋安 饰）是护龙山庄的四大密探，各自有着江湖众人无法匹及的绝技，效命于神侯朱无视（刘松仁 饰）。此时因正德皇帝(邓超 饰)痴傻，政局混乱，东厂趁乱把持朝政，迫害忠良。权倾朝野的东厂太监总管曹正淳(李建义 饰)更是仗势欺人，处处与护龙山庄为敌。正在四人深陷于儿女情长和身世经历之时，当前形势急剧恶化，一件又一件命案直指护龙山庄，无形的阴谋笼罩着护龙山庄，神侯更是因为被奸人诬陷锒铛入狱。而神侯此时趁机设局假死，在四大密探联手合作下将曹正淳一伙连根除去。原本以为阴谋就此结束的四人此时却发现，原来皇帝的痴傻实为伪装，而身边最大的阴谋家正是养育他们长大，却窥探皇位二十多年的神侯！
