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Longtime screenwriter Yau Nai-hoi makes his feature directorial debut with this tale of a Hong Kong surveillance unit ensnared by underworld violence. In the aftermath of a daring daytime jewelry store heist, criminal mastermind Shan (Tony Leung) and his crew celebrate their success with a rooftop barbecue. The good times are quickly cut short, however, when Shan's men summon the wrath of their boss by attempting to muscle him into giving up a bigger cut. But Shan's men aren't the only ones who want a heartier piece of the pie, because it seems as if the boss himself is scheming to take the men at the top for all they're worth. In order to do so, Shan executes a second high-profile robbery that finds him doggedly pursued though the streets by determined cops. In order to catch the elusive criminals the police will have to work harder than ever, a fact that's not lost on grizzled surveillance unit leader Captain Wong (Simon Yam). Known casually by the code name "Dog Head" to his underlings, Captain Wong is currently mentoring pretty rookie "Piggy" (Kate Tsui), and has made it unmistakably clear that the sole function of the SU is not to engage the criminal, but observe him. 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.