The series is set in China during the Ming dynasty. Zhu Wushi, the Emperor's uncle, had received an order from the previous emperor to establish a secret service called "Dragon-Defending Mountain Manor" to protect the monarchy. Zhu Wushi has four elite secret agents serving under him. They are known by their code names: "Heaven Number One", "Earth Number One", "Mysterious Number One", and "Yellow Number One". "Heaven Number One" is the calm and cool-headed Duan Tianya, who was raised by Zhu Wushi as a godson and sent to Japan to learn ninjutsu from the Iga School. While in Japan, he had a romantic relationship with Yukihime, the elder daughter of Yagyū Tajima-no-Kami, the patriarch of the influential Yagyū family. However, he becomes an enemy of the Yagyū family after taking revenge against Yukihime's brother, who had murdered his sensei. Torn between her family and her lover, Yukihime ultimately sacrificed herself to prevent her father from killing Duan Tianya in a duel. Years later, Duan Tianya encounters and starts a relationship with Yukihime's sister. He gets into conflict with the Yagyū family again. "Mysterious Number One" is the beautiful and intelligent Shangguan Haitang, who is well versed in various skills and arts, including medicine and astronomy. Like Duan Tianya, she was also raised by Zhu Wushi as his goddaughter. She helps her godfather manage and run the day-to-day affairs of the secret service. "Earth Number One" is the cold and aloof Guihai Yidao, whose skills in using the dao are unparalleled. He desires to avenge his murdered father, and sees it as his raison d'être. In his quest for vengeance, he masters a powerful but highly perversive skill, and finds himself falling under its evil influence — to the point where he loses control of himself and starts to indiscriminately kill anyone who tries to stop him. He has also been secretly in love with Shangguan Haitang for many years but does not have the courage to confess his feelings to her. "Yellow Number One" is the streetwise Cheng Shifei, who used to be a gambler, thief and trickster. By coincidence, he encounters Zhu Wushi's rival Gu Santong and inherits his inner energy and powers, including a skill that allows him to temporarily turn his body into impenetrable metal. He also meets the Emperor's sister Princess Yunluo, starts a romance with her and marries her eventually. The Dragon-Defending Mountain Manor has an ongoing political rivalry with the Eastern Depot, another imperial spy agency led by the eunuch Cao Zhengchun. The Emperor knows that Zhu Wushi and Cao Zhengchun are equally influential in politics, so he has been using them to balance against each other and prevent either side from becoming too powerful. The four agents help Zhu Wushi to counter Cao Zhengchun and the Eastern Depot, as well as protect the Emperor from domestic and foreign threats. Towards the end of the series, Zhu Wushi overcomes and eliminates Cao Zhengchun, and becomes the single most influential person in the imperial court. He reveals his true colours: all these years he has been wanting to usurp the throne from the Emperor, and now he is in the best position to do so. The four agents combine forces to defeat their treacherous master and save the Emperor.
Shooting for The Royal Swordsmen started on 22 March 2004 in Wuxi, and wrapped up on 21 June in the same year.
Broadcasts
The series was first aired in mainland China on Shenzhen Media Group's Drama Channel on 30 January 2005. It started airing on 14 February 2005 on CTS Main Channel in Taiwan, and on 25 October 2006 on TVB in Hong Kong.
