Eye in the Sky (2007)

Also known as "Gun Chung", this crime thriller stars Simon Yam as Sergeant Wong Man-chin, the head of the SU (Surveillance Unit), a dedicated unit of cops whose job is to follow, listen, and learn - but they must never engage their quarry. When "Piggy" breaks the primary rule of the SU during a particularly sensitive mission on the streets of Kowloon, Shan retaliates by striking back at the unit with everything he's got.