Is it really a coincidence that there are so many conspiracy thrillers at the moment? The Shadow Line, which over seven episodes explains the murder of a criminal who seemed to have state protection, starts this week on BBC2, while the American drama Rubicon, in which newspaper crossword clues hint at a shadowy group manipulating American history, runs on BBC4.
And currently in production in Belfast is another BBC series about hidden forces within Britain: Ronan Bennett's Undisclosed, with Philip Glenister as a solicitor whose investigation of his brother's death reveals, according to early publicity, a "conspiracy that reaches deep into the heart of the political system".
That description doesn't give much away because it is almost a definition of what a conspiracy thriller does. It certainly applies to The Shadow Line, the title of which refers to the blurring of morality between the police, led by Chiwetel Ejiofor, and a drug gang, fronted by Christopher Eccleston, as they simultaneously investigate the slaying of a man who'd mysteriously been granted the establishment's judicial trump card: the "royal prerogative of mercy", which gives immediate release from jail and wipes clean a criminal record.
Hugo Blick, who is writer, producer and director of The Shadow Line, says: "At first, The Shadow Line looks like a police procedural, but that was its least interesting aspect to me. And what it evolves into is, absolutely, a conspiracy thriller." Blick was also drawn to a particular subsection of this dramatic category: the espionage thriller, typified by Tinker, Tailor, Soldier, Spy, to which parts of the plot respectfully nod.
There's little mystery about why conspiracy plots are so popular. Human life is prone to contain unfair surprises - from the sudden death of loved ones to a general election, sporting encounter or job interview that produces an outcome we dislike - and our brains seem programmed to seek consolation in the fact that the result was rigged.
Public conspiracy theories tend to flourish at times of maximum suspicion of governments and big business, which is why, in this time of high international military and economic tension, they are reflected so heavily in drama at the moment.
Blick has previously been most associated with the TV monologue - writing and directing Marion and Geoff, Sensitive Skin and The Last Word - but his latest project has more than 70 characters: it would be very difficult to write a conspiracy thriller as a soliloquy because the structure demands complexity.
None of the more than six dozen participants in The Shadow Line will be played by Blick, although he has form as an actor, including the younger version of Jack Nicholson's Joker in Batman. Busy with his three behind-camera roles, Blick has resisted the temptation to give himself a cameo role, although he jokes that "the experience of having blown away Batman's parents" makes him a better director of performers.
As the director Ken Loach spelled out in the title of his movie example of the genre, such conspiracy dramas seek to expose a Hidden Agenda. And, in the names given to significant past TV examples of this form, the political nature of these agendas is clearly declared or, rather, stated.
Brian Phelan's In the Secret State (1985), State of Play (2003), written by Paul Abbott, and The State Within (2006), with scripts by Lizzie Mickery, all announce from their first appearance in the schedules an intention to slice through the protective screens of the establishment.
Blick's title alludes most directly to Edge of Darkness (1985), which he cites as a personal favourite, in which policeman Bob Peck's investigation of the death of his anti-nuclear-campaigning daughter revealed US-UK military and industrial collusion.
Lizzie Mickery also acknowledges that drama as an influence on The State Within: "The reason it was superb was that a personal quest led into a huge public issue: the perfect conspiracy thriller."
Such fictions concern what happens in the shadows of the establishment world and Blick's story, true to its title, often literally takes place in dark corners and heavily shadowed locations: "I had long discussions with the cinematographer about that," he says.
"I wanted that look because it suits the atmosphere of these stories to have things that can't be seen but, also, it's a way of saying to the viewer: don't look there, look here. There's a tendency in film and television to light everything in view and I think that's because they often don't know where the story is."
In a culture at any one time, there exist two sets of conspiracy theories: the fictional and the factual. The difference is that the theoretically "real" ones - that the Moon landings were faked or that Princess Diana and the weapons expert Dr David Kelly were murdered - tend to be more implausible than those that admit to being made up.
It's a useful rule that two questions should always be asked about any historical counter-version: who would benefit? And how would it have been done?
In this respect, the American government certainly had a good motive to stage the Apollo missions (bragging rights over the Soviet Union at far less expense) but would have had to buy the silence of thousands of employees and persuade Neil Armstrong and the other astronauts to live a lie.
And, while a decent MI5 operative could have made the murder of Dr Kelly look self-inflicted, his sudden death turned a small political crisis into a major one, meaning that the government would not have gained from having him whacked.
Similarly, the assassination of Diana would have been logistically almost impossible (given last-minute switches of cars and routes) and also self-defeating: she was a much bigger threat to the royals as a dead saint than the live girlfriend of a playboy.
It's certainly no coincidence that the greatest Hollywood examples of the genre grew out of the period during which public events - the murders of Jack and Bobby Kennedy and Martin Luther King, the Vietnam War and the Watergate scandal - made Americans deeply suspicious and uncertain about the direction of the country.
Sydney Pollack's Three Days of the Condor (1975), and two films directed by Alan J Pakula - The Parallax View (1974) and All the President's Men (1976) - all implicitly or explicitly dealt with the fear of a coup d'etat in America, engineered either by rogue CIA operatives, shady corporations or Richard Nixon.
Those 70s paranoid classics influenced The Shadow Line, Blick acknowledges. "It was a great period for the conspiracy thriller, because American society was in a spin; they didn't know where they were at. So big entertaining mainstream movies were made with subjects and ideas that would previously have been left-field."
It's equally revealing that Hollywood film versions of Edge of Darkness and State of Play were both released just after the administration of George W Bush, in which, according to conspiracy theorists, the neocons, led by Vice President Dick Cheney, ran an all-powerful shadow administration beneath the surface of the elected one.
Lizzie Mickery is clear that The State Within came out of the same political atmosphere: "It was just after Iraq and we were very sensitive to the idea of government and big business in conspiracy with each other."
The Shadow Line follows the tradition of conspiracy thrillers in beginning with a murder. But there's no collusion in this: a plot featuring a cover-up will almost of necessity involve a secret big enough to require the final silence of those who may disclose it.
In his exploration of a world of secrecy, Blick has the advantage over earlier practitioners in this area of the increasing possibilities, for communications and surveillance, of the web and mobile phones: "I carefully researched the triangulation of cell-phone signals and so on. But I think the trick is to take the narrative possibilities of technology without becoming a slave to it."
The most crucial aspect of a conspiracy thriller is the ending, when the revelation of what was really going on is expected to resolve the accumulated ambiguities and tension. An audience is likely to be disappointed if the conspiracy is too large - as in Oliver Stone's JFK, where the premise depended on the absolute discretion of thousands of people - or too small, causing viewers to think: it was all for that?
Intriguingly, both Edge of Darkness and State of Play went into production before final scripts were settled. In the former, the Bob Peck character was initially intended by the scriptwriter to turn, in an ecologically symbolic moment, into a tree (a denouement from which creator Troy Kennedy Martin was dissuaded), while, in the latter, David Morrissey, whose character had seemed in the early scripts to be the dupe or hero, eventually discovered that he was the villain.
Avoiding these risks, Blick spent four months pre-plotting his drama, from beginning to end, creating a jagged diagram of interlocking storylines on whiteboards. "A good conspiracy thriller must end somewhere disquieting, that unsettles the audience."
Lizzie Mickery also recalls having whiteboards covered in graphs for The State Within: "I woke up at 3am one day, thinking 'Oh God, does the plot work?', and got up and went through it all."
It would be suicidal for the prospects of his series for Hugo Blick to reveal at this stage what the solution to The Shadow Line is, but he promises: "What is finally revealed is something that affects you and me directly. In common with most conspiracy thrillers, it is a warning tale. I just edited the final part of the final episode and it shocked me."