Saying al Qaeda isn't some huge and well-defined organisation isn't the same as saying there's no danger, though. Adam Curtis, creator of the programme, spelled this out in more detail in a Q&A session on the BBCs site:
Are you saying that there is no threat?
No, the series did not say this. It was very clear in arguing that although there is a serious threat of terrorism from some radical Islamists, the nightmare vision of a uniquely powerful hidden organisation waiting to strike our societies is an illusion.
As the films showed, wherever one looks for this "al-Qaeda" organisation - from the mountains of Afghanistan to the "sleeper cells" in America - the British and Americans are pursuing a fantasy.
The bombs in Madrid and Bali showed clearly the seriousness of the threat - but they are not evidence of a new and overwhelming threat unlike any we have experienced before. And above all they do not - in the words of the British government - "threaten the life of the nation". That is simply untrue.
Are you saying it's a conspiracy?
No. The use of fear in contemporary politics is not the result of a conspiracy, the politicians have stumbled on it. In a populist, consumerist age where they found their authority and legitimacy declining dramatically they have simply discovered in the "war on terror" a way of restoring their authority by promising to protect us from something that only they can see.
I don't think it will last. Already senior parts of the Establishment are beginning to question the very basis of the politicians' argument - that "al-Qaeda" is a threat like no other which "threatens the life of the nation".
Do you believe it possible that the American Neo-Cons engineered the 9/11 atrocity as a catalyst for their program?
No