Narcotics officer Shaolun (Tony Leung Ka Fai), in order to bring down the local drug kingpin King (Michael Wong), forces a female street thug with an equally sketchy past, Coco (Rosamund Kwan) to infiltrate King's organization and provide information. What no one expected was for both men to fall for Coco, and Coco for King.

Despite negative psychological evaluations, Mike Wong is assigned as the captain of the SDU, a special task force linked to the Customs Department. Their mission: apprehend nasty drug dealers, led by the “Ice Queen.” To do this, Mike has to team up with customs cop Minnie. Despite their clashes, the two learn to understand each other, but not before things get bad.

A look at the SDU in the Royal Hong Kong Police Force - how members are selected, how they are trained, their lives, what they think, the relationships between SDU members and their friends and the pressures they face.