The series is set in 17th-century China during the
Ming dynasty. Zhu Wushi, the Emperor's uncle, had received an order from the previous emperor to set up a
secret service to protect the monarchy. One of the most formidable martial artists in the
jianghu himself, Zhu has recruited four elite agents who are known by their code names: "Heaven One", "Mysterious One", "Earth One", and "Yellow One". "Heaven One" is the calm and cool-headed Duan Tianya, who was raised by Zhu as his godson and sent to Japan to learn
ninjutsu from the
Iga School. While he was in Japan, he had a romantic relationship with Yukihime, a daughter of
Yagyū Tajima-no-Kami, the patriarch of the influential
Yagyū clan. He also met
Nemuri Kyōshirō and learnt
kenjutsu from him. However, he became an enemy of the Yagyū clan after taking revenge against
Yukihime's brother, who had murdered Nemuri. Torn between her family and her lover, Yukihime ultimately sacrificed herself to prevent her father from killing Duan in a duel. Years later, Duan encounters Yukihime's sister and starts a romantic relationship with her, drawing him into another conflict with the Yagyū clan. "Mysterious One" is the beautiful and intelligent Shangguan Haitang, who is well-versed in various skills and arts, including medicine and astronomy. Like Duan, she was raised by Zhu as his goddaughter and trained by him in martial arts. She is in charge of an organisation made up of experts who are considered "Number Ones" in their respective trades, fields, and professions. Through the organisation, she helps her godfather gather intelligence and scout for talents to join the secret service. "Earth One" is the cold and aloof Guihai Yidao, whose
swordsmanship is unparalleled. He desires to avenge his murdered father and sees it as his purpose in life. In his quest for vengeance, he masters a powerful but highly perverse 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 for many years but does not have the courage to confess his feelings to her. "Yellow One" is the streetwise Cheng Shifei, who used to be a gambler, thief, and confidence trickster. By chance, he encounters a dying Gu Santong, Zhu's rival who had lost to Zhu in a duel and has been imprisoned for many years. Before his death, Gu transfers to Cheng all his
neigong and knowledge of martial arts – including a skill that allows him to temporarily turn his body into impenetrable metal – in the hope that Cheng will help him fulfil his wish of defeating Zhu. Cheng also incidentally meets the Emperor's sister, Princess Yunluo, and marries her eventually. The secret service has an ongoing political rivalry with the
Eastern Depot, another imperial spy agency led by the
eunuch Cao Zhengchun. The Emperor knows that Zhu and Cao are equally influential in politics, so he has been pitting them against each other in order to prevent either side from gaining too much power. The four agents help Zhu counter Cao's machinations on one hand, and perform their duties by protecting the Emperor from domestic and foreign threats on the other hand. Zhu ultimately defeats and eliminates Cao, becoming the single, most powerful figure in the imperial court. At this point, he finally reveals his true colours. All these years, he has been secretly plotting an elaborate scheme to usurp the throne from the Emperor, and he is now in the best position to do so; the four agents are merely his pawns. In the end, the three surviving agents combine forces to defeat their treacherous master and save the Emperor. == Cast ==