It's storylines are strictly fictional. But the hit show Spooks has helped to bring about the creation of a real intelligence agency in America.
Advisers to George W Bush are said to have used the BBC TV series, which is called MI-5 in the U.S and has a cult following there, to hammer home the message that a domestic spy agency is vital for the war on terror.
Now the U.S. has launched its own version of MI5. It will be known as the Defence Counterintelligence and Human Intelligence Centre.
After the September 11 terror attacks, Mr Bush made it clear that he admired the way the UK's intelligence agencies work - with MI5 operating on home soil and MI6 abroad. The US has masses of intelligence agencies, but it has always been illegal for them to operate domestically.
The CIA has traditionally done the bulk of foreign intelligence work leaving the FBI to chase home-grown terrorists and spies working out of foreign embassies. Critics said the FBI was overstretched by its normal criminal work and had missed clues that could have prevented the 2001 terror attacks.
The formation of the new service follows years of behind-the-scenes wrangling between the White House, Congress, and civil liberties groups.
As with MI5, the agency's main job will be to gather intelligence spies or terrorists, and force them to leave U.S. soil if they are diplomats - not to prosecute them.
Its formation is part of a flurry of intelligence changes ordered by Mr Bush during his final months in office.
Toby Sullivan, director of counterintelligence, told reporters at a Pentagon briefing: ‘These sensitive, clandestine operations are tightly controlled departmental activities run by a small group of specially selected people.’
As with MI5, he said the new agency's main job will not be to prosecute spies or terrorists, but to gather intelligence on them, and force them to leave American soil if they are diplomats.
Mr Sullivan said: ‘We are an arrow in somebody's quiver. We identify the possible threat; and we work with those who are feeling the focus of the threat.’
Prosecutions would be pursued by the FBI in the same way as MI5 uses Special Branch.
The new service operates under the US Defence Department umbrella.