Peter Taylor's documentaries tend to induce stomach ache and queasiness. I mean this as a compliment. In a world where the word information calls to mind Coleridge's "Rime of the Ancient Mariner" ("Water, water, everywhere,/ Nor any drop to drink"), Taylor never fails to deliver thirst-quenching new stuff that matters. How does he do it? His latest series, The Secret War on Terror (14 and 21 March, 9pm), is so crammed with news lines that the BBC has imposed embargoes. The other night, he made a special guest-star appearance on the Ten O'Clock News. He was inside Guantanamo Bay, which President Obama will not be closing after all. "Unedifying" was the adjective he used to describe some of what has gone on there, and it departed his lips with the solemnity of Moses delivering the Ten Commandments.

The Secret War on Terror asks two questions. First, to what degree has the west sacrificed its democratic principles in the fight against terrorism? Second, has this struggle made us any safer? The news is not, I'm afraid, good. We've known for a while about extraordinary rendition and torture. I asked Tony Blair about both when I interviewed him a couple of years ago (needless to say, he did not fall on his knees and beg for God's forgiveness; his attitude was akin to that of a parent addressing a vexatious teenager who would only finally grasp "the truth" when she was fully grown up).

Nevertheless, Taylor's charge sheet was shocking. Which was more horrifying? Eliza Manningham-Buller, the former head of MI5, admitting in her first televised interview how "shocked" she was to realise, shortly before her retirement, exactly what the Americans had been up to? Or the conversation in which Pat D'Amuro, former assistant director of counter-terrorism at the FBI, told Taylor that, in 2002, he had instructed his operatives to return from Pakistan, where they were interrogating the alleged al-Qaeda terrorist Abu Zubaydah, because the CIA had arrived and its officers were planning to use "enhanced interrogation techniques"? (D'Amuro did not think that history would judge him kindly were he to turn a blind eye to torture, for which this is the received euphemism.) I think that the lowest moment may have been when Pervez Musharraf, the former president of Pakistan, denied that the UK government had ever requested his regime to refrain from using torture and then giggled, as if he was discussing a childish game.

“The banality of evil" is a phrase much overused, but I can't think of a better one in this instance. The Bush administration treated the map of the world as a kind of torture brochure. "If you want good intelligence, send him to Syria; if you want him to disappear, send him to Cairo," said one spook. And Morocco? Oh, Morocco. The treats in store. "You can get what you want in Morocco." Hell, those guys even pull fingernails out. Meanwhile, Donald Rumsfeld was scribbling in the margins of classified documents. Why shouldn't detainees have to stand for longer than four hours at a time? The Rumsfeldinator can stand for six to eight hours a day!

Taylor will examine our national security in part two and I don't expect to feel reassured. It's not only that torture produces unreliable information (Khalid Sheikh Mohammed, the monster who beheaded the US journalist Daniel Pearl, was waterboarded 183 times in one month, with the result that about 90 per cent of the crimes he admitted to were fabrications). It's also a huge propaganda boost for those who seek to radicalise. Taylor asked Manningham-Buller, wearing pearl earrings and pastel tweed, if torture damages the west. She was unequivocal: it does. Her analysis was far more brief and less impassioned than that of the Reprieve lawyer Clive Stafford Smith. But it had twice the impact. She was a spook, after all. She knows stuff. Taylor clearly recognised this. He nodded his head but kept shtum. There is a time to push and a time to fall silent. That he knows this is one of the things that set him apart.