Ted Sharp, known to many as Sharky, is a part-time agent of British Intelligence.
The exact identity of that organization is not specified but it is shown to work very closely with standard law enforcement groups and its objective seems to be countering the connection between organized crime in the U.K. and terrorists around the world. Certainly the people that Sharky comes into contact with and deals with affiliated with this agency are either still policemen although there is that one stranger in the initial meeting who clearly admits to one and all, with "a broad flashing smile", that "I'm a spook".
Sharky is not a spook. He is a cop. He has been for 25 years and would probably have liked to stay one for another half decade to reach his maximum allowed time in service but while he was a very good detective with a pretty impressive record, he knew - and was told straight out - that he had advanced as far as he ever would. Sharky was a valued cop but he did not play the political game whatsoever and so had made some people higher up either unhappy or at best nervous.
What brings Sharky into the cloak-and-dagger world is his search for his missing adult daughter, Christine, who had chosen some time back to transfer from the police world into the shadowy world and then suddenly gone missing. Sharky was like everyone else in believing the worst had happened to her and sadly accepted the loss but when she was sighted some time later by a credible source, nothing would stop him from finding the truth.
Once he had made the temporary transition from retired police detective to would-be spook, he discovered that success in that new life would mean that they would likely want to use him again.
Well, there is a small matter of Liz, a woman he had a brief relationship with years before who would be his liaison with the intelligence agency. While their interactions would not reach the intimacy it once did, she and Sharky would become good working partners.