“Surveillance” literally means “watching over”, or “eye in the sky” in French. It is an act of monitoring people and their behavior with trained skills and technological means.
 
CIB, or Criminal Intelligence Bureau, is the most secretive branch of Hong Kong police. Its agents appear like ordinary people and possess the skills of blending into their surroundings to conduct surveillance on targets.
 
CIB CAPTAIN HUANG (Simon Yam) leads rookie BO (Kate Tsui) and fellow agents to tail a suspected armed robbery ring. They quickly discover the criminals’ plan to commit a jewelry heist. The investigation also leads CIB to focus on an invisible mastermind behind the plot.
 
CHEN (Tony Leung Ka-Fai) has the look of a soft-spoken bookworm, but he is actually a professional criminal who excels in eluding the police. Alerted by Captain Huang’s team closing in on him, Chen decides to call off the heist.
 
But Chen’s decision comes too late. Captain Huang manages to trace the mastermind’s call and locate his whereabouts. The armed robbers are killed and Chen is forced to go into hiding after brutally killing a cop.
 
Chen’s disappearance throws the investigation into a dead end. CIB is assigned to a new kidnapping case. Bo realizes her mistake during the operation cost CIB the arrest. She wonders if she has what it takes to become a good surveillance agent.
 
While staking out her new target, Bo spots Chen who is getting ready to leave town. Bo immediately gives pursuit with Captain Huang backing her up. But out of nowhere Chen attacks Captain Huang. Lying in his own pool of blood, Captain Huang tells Bo not to give up.
 
The pouring rain makes it all the more difficult for Bo to find Chen. Just when all hopes seem to be lost, the rain stops. Before Bo knows it, the criminal mastermind reveals himself amongst the crowd...