OF COURSE TORTURE is the official policy of the United States government. For decades, the CIA has been instructing foreign nationals on the fine points -- if that's the proper term -- of inflicting agony. Some of the most horrific sequences in On Company Business, Allan Francovich's 1980 documentary, detail the techniques taught to the Shah's secret police by the boys from Langley. What's changed since 9/11 is that Americans no longer keep their distance. Top officials, and their op-ed echo chamber, insist that waterboarding and other torments are necessary and justifiable. Yet they decline to take the rap when someone like Dilawar, the taxi driver at the center of Alex Gibney's crucial new film, turns up dead.

Dilawar's story is inevitably somewhat sketchy, but there's no plausible alternative to the version offered by Taxi to the Dark Side: Nabbed by one of the many Afghans who sold prisoners to the Yanks for cash, the young man became a PUC -- person under control -- in 2002 at the U.S. prison at Bagram Air Base. He was not charged with any crime, and had no known ties with Al Quaeda or the Taliban. He probably wouldn't have been transferred to Guantanamo, and most likely would have been released in short order. Before that could happen, however, he died from being severely beaten and then hung by shackles from a metal-grate ceiling, which aggravated his injuries.

Gibney, who directed 2005's semi-pointed Enron: The Smartest Guys in the Room, finds many of the people who know Dilawar's story best. He marshals interviews with one of his fellow inmates, two New York Times reporters who followed the case, and several U.S. servicemen, some of whom were court-martialed over the death. (The charge? Not murder, but "dereliction of duty.") They are frank about what happened to Afghani prisoners, and why. The top brass "wanted them to be guilty," says an interrogator who thought Dilawar was not involved in rocket attacks, the ostensible reason for his incarceration.

On the pro-torture side, no one defends what happened to Dilawar. Such notorious figures as Capt. Caroline Wood, who was commanded interrogators at Bagram before being sent to Abu Ghraib, are not talking. But Gibney does gather the comments of American torture's top contemporary defenders, including George W. Bush, Donald Rumsfeld, and apparent mastermind Dick Cheney, who inspired the movie's title: On Meet the Press in 2001, he explained that to defeat Islamic-extremist violence, the U.S will have to enter "the dark side." No one can say Cheney isn't true to his sinister word.

While circling back to Dilawar, the film conducts a thorough examination of the issues raised by one cabbie's fate. Bagram's harsh techniques expand to Abu Ghraib and then Guantanamo, and torture is central to the process: Those hoods placed over prisoner's heads, for example, are not security precautions but sensory-deprivation devices, designed to undermine the wearer's personality. The argument is persuasively made -- notably by longtime F.B.I. interrogator Jack Clooney -- that the information elicited under extreme pain is useless. The fabled connection between Iraq and Al Quaeda is just one of the phony scoops extracted by torturers.

A harsh irony of the post-9/11 quest for "intelligence" is that the top officials extolling the process don't especially want new information. They're content with their preconceptions, plus a few new half-truths for the propaganda mills. It's left to films like Taxi to the Dark Side to reveal what's actually happening, a service to the American public that -- to judge by the reaction to previous "war on terror" docs -- will probably be underappreciated. If most people would rather watch 24, a show whose glamorization of torture Gibney analyzes, that's just a matter of taste. Except for the future Dilawars, whose lives depend on turning U.S. policy away from the dark side.