The young Dee Renjie (Mark Chao) arrives in the Imperial Capital, intent to become an officer of the law. Empress Wu (Carina Lau), who is at the start of her reign, has sent the fiery-haired Detective Yuchi (Feng Shaofeng) to investigate a sea monster that stalks the city at night. By chance, both Dee and Yuchi fight off the creature when it attacks a ceremonial procession carrying the beautiful courtesan Yin (Angelababy) to be sacrificed at a temple to appease the Gods. Suspicious of the stranger, Yuchi has Dee imprisoned. Dee escapes with the help of visiting doctor Shatuo (Lin Gengxin) and together they discover that the upper reaches of society, including the Emperor himself, are addicted to a poisonous tea produced by Yuan's (Kim Bum) famous teahouse. Yuan, who was romantically attached to Yin, has been missing for months. Dee shadows the courtesan, who is under Yuchun's protection, until the sea monster reappears. Just as the two men are about to kill it, Yin reveals that the monster is her lover, Yuan. Yuan has been poisoned by a foreign Prince whose kingdom was overthrown by the Emperor. As the Prince's army readies itself on a nearby island to attack the capital, a real sea dragon rises from the deep.
