Dee Renjie sets out to solve his very first case in director Tsui Hark's prequel to the 2010 hit DETECTIVE DEE AND THE MYSTERY OF THE PHANTOM FLAME. The bustling capital city of Luoyang has come under attack from a powerful force from the depths of the ocean, threatening the entire Celestial Empire. Dispatched by Minister Yan to investigate for the Da Lisi, Detective Dee (Mark Chao) quickly learns that two dragons have appeared in the capital. Meanwhile, the newly crowned Empress Wu (Carina Lau) has ordered Detective Luchi to investigate as well. Later, the sea monster attacks a ceremonial procession, spreading a wave of chaos throughout the city. When Dee uncovers evidence of a conspiracy implicating the most prominent members of the Imperial Family, he is imprisoned for treason. Now Dee's only hope for exposing the truth behind the bizarre events unfolding in Luoyang is to investigate the radiant courtesan Yin (Angelababy), but in order to do that he'll first need to escape prison with a little help from Shatuo Zhong ($Lin Gengxin). Once Dee is free, the race is on to save the city, and expose the treachery that threatens to topple the empire.