The story begins with a group of organised burglars successfully robbing a jewelry store. Wet-behind-the-ears new recruit Piggy is plunged at the deep end in her first surveillance assignment working alongside her mentor, Sergeant Wong. They work for the Hong Kong Police Force's Criminal Intelligence Bureau. After reviewing the jewelry store surveillance videos recordings, the force attempts to track down one of the robbers by staking out the neighborhood they think he lives in. Fatman is identified and tracked without his knowledge through a combination of tails, surveillance footage, and data-mining, including accessing his Octopus card. The force ambush is established at the scene of the next robbery, but the elusive Hollow Man notices the police. As the robbers flee, some are killed by a Police Tactical Unit, but the rest escape. Chan kills a uniformed police officer as he flees, and Piggy stops to give first aid rather than continue the pursuit. The surveillance unit is then called to another case; a kidnapping. Piggy is staking out a phone booth where the kidnapper may make a call, when she sees Hollow Man go by. She recognises the kidnapper from Fatman's apartment building and allows the police to rescue the kidnap victim, while she pursues Hollow Man. After a tense confrontation in a cafe, where Hollow Man confronts her, she escapes suspicion, but Hollow Man notices Sergeant Wong and stabs him in the neck with a pair of scissors. Wong convinces Piggy to continue to follow Hollow Man as he slowly bleeds to death. Piggy follows Hollow Man to his hideout and calls for the Police Tactical Unit again. Meanwhile, Wong survives his injury and gets medical attention. In the raid on the hideout, Hollow Man flees down a dock and wounds himself mortally in the neck running past a hanging hook, and the other robbers are captured.
Critics and fans hailed the plot's dark atmosphere and realistic use of surveillance tech to make the story believable. But the film, however, has been criticized for having not enough creativity for those looking for something more. It has also been criticized for being too short. Perry Lam of Muse magazine writes, 'If the ending of the movie is more than a little contrived and predictable, there are enough surprises along the way for us to savor before we get there.'
