The Weekend's TV: The Conspiracy Files: 9/11 &ndash; The Third Tower, Sun, BBC2<br />George Gently, Sun, BBC2

Imagine an infection that actually feeds on the only antibiotic available to counter it and you have some measure of the problem in dealing with conspiracy theories. When conspiracists encounter solid evidence against their favoured scenario, they have only two options available to them. They can compromise their religious belief in the narrative they've constructed, or they can simply enlarge it, extending the conspiracy so that, amoeba-like, it absorbs this irritating foreign body and digests it whole. Almost invariably, they choose the latter, and The Conspiracy Files: 9/11 – the Third Tower, a film about the collapse of World Trade Centre Building 7, offered several choice examples of the process. What conspiracists believe about Building 7 is that it was deliberately destroyed, brought down by a pre-planned explosion so that the evidence proving government complicity in the events of 9/11 would be shredded. They base their conviction on footage of the collapse, in which it does indeed look as if the building has been brought down by controlled demolition. They then shore up this ill-informed impression with oddities picked from the rubble of that day – passing remarks, inaccuracies in the news coverage, fragments of debris – all of them carefully sifted and separated from their context so that their larger meaning is obscured. And woe betide you if you suggest that the resulting collage is implausible in any way.

Mark Loizeaux did, a demolitions expert who pointed out that preparing a large skyscraper for demolition takes months of work and requires a cat's cradle of wiring all over the building. Not only that, but the amount of explosive required to bring down Building 7 would have shattered windows for hundreds of yards around, whereas the only windows broken in nearby buildings were those directly exposed to falling debris from the Twin Towers. A large chunk of grit for any amoeba to swallow, you would have thought, but rippling weirdly around the edges, that's just what happened. Richard Gage, founder of the ironically titled Architects & Engineers for 9/11 Truth, speculated that either specially concealed charges had been laid when offices were remodelled or that they had actually been incorporated into the building when it was first built in the 1980s, the government showing an impressive grasp of forward planning in the field of nefarious skulduggery. Other conspiracists took a simpler route: if Loizeaux was undermining their theory, then he must be part of the conspiracy, too. Indeed, now they thought of it, nobody was better qualified to have done the job. Loizeaux and his employees became the object of a hate campaign.

As did Jane Stanley, a BBC correspondent at the time of the attacks who became notorious to conspiracists because she discussed Building 7's collapse in a two-way with the studio in London at a time when it was still visible over her left shoulder. This is another cherished "smoking gun" for the cranks, and one that's nicely illustrative of their thinking. Asked what she could tell viewers about the reported collapse of Building 7, Stanley replied, "Well, only what you already know. Details are very, very sketchy." She was thinking on her feet, she explained, having been confronted with a statement that she had no way of checking. She described it as "a very small and very honest mistake", which wasn't quite true, since the "very honest" response to the original question would have been "I can't tell you a bloody thing about any collapse because this is the first I've heard of it, and frankly I don't know which way is up right now". One wishes BBC correspondents would occasionally adopt this degree of candour, but habits die hard and the engrained instinct is to conceal your ignorance rather than advertise it. So you have a choice: either the BBC had inadvertently revealed that, in concert with other broadcasting organisations, it was working from a prearranged script drawn up as part of the biggest conspiracy in world history, or a flustered reporter did the best she could in the middle of a breaking story. Probability of the former, vanishingly small; probability of the latter, approaching certainty, and yet if you opt for flustered cock-up, the conspiracists will dismiss you as a hopeless dupe of the new world order. Incidentally, if you're minded to send me a long email denouncing my gullibility and directing me to websites offering incontrovertible proof that Dick Cheney did it, don't bother. As you may already have guessed, I'm in on it, too.

It's not entirely surprising that people make up their own thriller stories if the commercially available ones are as dull as George Gently, a Northumberland-based detective drama in which Martin Shaw takes the lead. Why Gently? Because George Growly would be a bit too obvious even for a drama this committed to genre expectations. Peter Flannery's script gives some good, taut lines to the sidekick, the only character with space enough to surprise you, but it wasn't enough to stave off sleep.