In 2002, a young Afghan cab driver picked up some passengers near his home and drove off. He never returned to his village. He was captured, incarcerated by the Americans as a suspected terrorist, beaten to a pulp and found dead in his cell. Later his body was given back to his village and buried there, inspiring hatred for those who killed him.

That’s the starting point of Alex Gibney’s impressive, Oscar-winning documentary, which accuses the Bush administration of appalling mistreatment of prisoners in Afghanistan, Abu Ghraib in Iraq and Guantanamo Bay. Gibney asks the awkward question whether or not it is right to torture a known terrorist for the greater good of those who might otherwise be blown up by him.

He quotes Vice-President Cheney saying: “We have to work through the dark side ... It’s going to be vital for us to use any means at our disposal, basically, to achieve our objective.” But though it is shocked and despairing about what happened, the film strives for fairness.

Gibney incorporates interviews with the families of dead or incapacitated prisoners, reporters, former government officials, interrogators and prison guards who tell the same story — that unacceptable means were used to obtain a questionable end, torture being unlikely to get to the truth. He suggests that, when the facts came out, only the lowest people on the ladder were punished.

Those who tacitly gave the orders, or deliberately obfuscated them in the minds of young troops who were only given a few weeks military training and then asked to interrogate suspects, remained free of opprobrium.

Dilwar, the Afghan taxi driver, regarded as almost certainly innocent even by those who caused his death, is the chief character in the film. But Cheney and Donald Rumsfeld figure prominently. According to Gibney, they systematically compromised the principles upon which the United States and all democracies were founded. This is a film everyone should see — its shocking evidence seems incontrovertible.