After Detective Dee (Mark Chao) cracked the case of the Dragon King in the capital, Emperor Gaozong (Sheng Jian) bestowed upon him the sacred weapon, the Dragon Mace. Empress Wu Zetian (Carina Lau) was jealous of this. In order to steal the Dragon Mace and frame Detective Dee, the Empress summoned a group of "Aliens" who knew magic to plot against Detective Dee, and ordered Yuchi Zhenjin (Feng Shaofeng) to lead the group. With the help of medical officer Shatuo Zhong (Lin Gengxin), Detective Dee successfully escaped the persecution of the "Aliens" and negotiated a settlement with Yuchi Zhenjin. At the same time, Shuiyue (Ma Sichun), an assassin from the "Aliens", discovered that an unknown force had appeared in the capital. As Detective Dee was dealing with Wu Zetian's ambush, the Tang Dynasty fell into a deeper crisis. The "Demon Sealing Clan" appeared with their magic skills, and a "demon-slaying" war was about to break out...