Set during the reign of Emperor Huizong in the late Northern Song Dynasty. The government department known as the "Department Six" customarily has full jurisdiction over all criminal investigations in the imperial capital. Department Six is well-staffed and operates in a highly disciplined manner. Their chief, Commandant Liu, ranks the investigators by clearance rate and dangles the prospect of an operator to be named one of the "Great Four".

The story begins with the country experiencing a significant increase in circulation of counterfeit coin currency, leading to growing unrest and instability. Official investigators from the Department Six, acting on a tip-off, rush in full force to apprehend a suspect who is trying to sell a coin die stolen from the imperial mint, only to find the suspect and the evidence being taken into custody by agents of a hitherto unknown secret service, known as the Divine Constabulary, which is commissioned by the Emperor himself.

This arouses jealousy in Commandant Liu, who openly fires Cold Blood, one of his best men. Liu secretly orders Cold Blood to infiltrate the Divine Constabulary to find a way to bring down the rival agency. Liu is unaware that his own establishment has been infiltrated by double-agents dispatched by the mastermind behind the counterfeit currency.

Despite being aware of Cold Blood's true mission, the chief of the Divine Constabulary seeks out Cold Blood and welcomes him into the agency. Once inside, Cold Blood is surprised to observe that the Divine Constabulary staff live and work together more like a family than a highly formalised professional security force. Despite their small size, the Constabulary works efficiently and effectively through a few individuals with very specialised skills which are useful in solving crimes. Cold Blood finds his loyalties divided, and things are made harder as he becomes tangled in a love triangle with two girls, one from each agency.

Cast
Deng Chao as Lengxue (Cold Blood)
Liu Yifei as Wuqing (Emotionless)
Ronald Cheng as Zhuiming (Life Snatcher)
Collin Chou as Tieshou (Iron Hands)
Anthony Wong as Zhuge Zhengwo
Jiang Yiyan as Ji Yaohua
Wu Xiubo as An Shigeng (God of Wealth)
Gui Gui as Dingdang
Cheng Taishen as Bushen (Sheriff King)
Sheren Tang as Jiaoniang (Aunt Poise)
Ryu Kohata as Cen Chong (Avalanche)
Michael Tong as Han Long
Tin Kai-Man as Jia San
Waise Lee as Prince
Bao Bei'er as Dalang (Big Wolf)
Tang Zhiping as Butou (Thunderclap)
Zhang Songwen as Qian Jian (Treasury Minister)
Fang Anna as Hudie (Butterfly)
Miao Chi as Dayong (Guts)
Xiang Tianran as Ling'er (Bell)
Liu Changde as Xu Feng