It is, perhaps, the greatest documentary never made.

Back in 1945, Sidney Bernstein, the chief of the Psychological Warfare Film Section of the Supreme Headquarters Allied Expeditionary Force, was commissioned to create the definitive documentary chronicling the liberation of the Nazi concentration camps. Bernstein’s aim was, in his words, to “prove one day that this had actually happened” and have it serve as “a lesson to all mankind as well as to the Germans.”

He eventually roped in his good pal, Alfred Hitchcock, to serve as the film’s supervising director. But the horrifying and heartbreaking footage of numerous concentration camps, shot by British, American, and Russian World War II soldiers as they were being liberated, became tangled up in a complicated web of politics and artistic rows. A magnificent new HBO documentary pulls back the veil on the making of German Concentration Camps: Factual Survey.

The eye-opening film-on-a-film, Night Will Fall, will premiere January 27 on HBO. It is narrated by Helena Bonham Carter, produced by Stephen Frears and Brett Ratner, and directed by Andre Singer, who serves as president of The Royal Anthropological Institute of Great Britain and executive produced the documentaries The Act of Killing and Werner Herzog’s Into the Abyss. It was done in concert with London’s Imperial War Museum, and took 18 months of poring over thousands of feet of film to trace the making of the unmade epic.

“When I first saw material, it was shattering to see; a horrific experience,” Singer tells The Daily Beast. “I’ve been in the film world a long time and seen lots and lots of footage and you think you’ll get anesthetized to it, but that isn’t the case. This is something that is once seen, never forgotten.”

Singer’s documentary opens with footage from German Concentration Camps: Factual Survey of the British 11th Armoured Division liberating the Bergen-Belsen concentration camp in Northern Germany on April 15, 1945. There, the Allied troops discovered a strange sight.

“Neat and tidy orchards. Well-stocked farms lined the wayside. And the British soldier did not fail to admire the place, and its inhabitants—at least, until he began to feel a smell,” says a narrator in voiceover.

The British soldiers found tens of thousands of emaciated prisoners inside the camp, many of which were on the brink of death by starvation. The camera lingers on piles of naked, skeletal corpses stacked several bodies high, as well as line after line of dead children. A total of 30,000 corpses were witnessed by Allied troops, according to the film. Singer managed to track down several British soldiers who were there, and some break down in tears recalling the horrors.

“It’s very difficult to describe,” recalls survivor Anita Lasker-Wallfisch, who was 19 when Bergen-Belsen was liberated. “You’ve spent years preparing yourself to die, and you’re still here. Every British soldier looked like a God to us.”

While Soviet intelligence had uncovered the existence of concentration camps in Poland as early as July 1944, because of the Soviet’s alleged record of falsifying atrocity reports, these warnings went largely ignored. However, once Bergen-Belsen was liberated, Bernstein decided to include footage of Russian troops liberating the Majdanek concentration camp in occupied Poland.

“We opened one warehouse. Women’s hair,” says Matvey Gershman, a member of the Soviet’s 8th Guards Army at Majdanek. “We opened the second warehouse. Children’s shoes. The third warehouse, something else: Zyklon gas in barrels. And ashes, ashes. They stored people’s ashes the way they stored women’s hair.”

Footage from Bernstein’s documentary shows Russian soldiers opening stacks of large packages the size of duffle bags and removing piles of human hair, as the camera rests on a young girl’s ponytail amid the pile in close-up. You see crematoriums filled with ashes, still burning, and long rows of human bones.

Also included is footage of the Russian liberation of Auschwitz, including a fascinating sequence wherein Soviet soldiers are captured clad in white, camouflaged snowcoats crawling across the wintry tundra towards the Nazi death camp while clenching ski poles, with skis attached to their feet. Eva Mozes Kor, a survivor of the sadistic Mengele experiments, tears up watching footage of her younger self being freed from captivity.

American soldiers, meanwhile, captured the liberation of Buchenwald and Dachau, and since American films began using 16mm color footage in January 1945, Bernstein’s film includes haunting color shots of heaps of withered corpses at Buchenwald, as well as shrunken Jewish heads inside jars. The American troops made local German citizens from the nearby city of Weimar visit the camp and witness the genocide firsthand. A German woman is seen fainting and being carried away, while other women casually turn their heads away in disgust.

“They had been aware of the camp and had been willing to make use of the cheap labor it provided—as long as they were beyond smelling range of it,” the narrator says.

While Bernstein’s opus was first commissioned in April 1945, it wasn’t until June that his pal, “The Master of Suspense” Alfred Hitchcock, came onboard after being released from his Hollywood duties. “I had felt that I needed at least to make some contribution,” Hitchcock says in an interview from 1962. “There wasn’t any question of military service—I was overage and overweight at that time—but nevertheless, I felt the urge.”

Since Bernstein was only given three months to complete the film, by the time June rolled around, he felt the need to recruit Hitch to help him piece it together. “Bernstein knew he needed some leverage,” says Singer. “He thought if they had a big name director it would help not just because it was better for the film, but it would give credibility to the film.”

So, Hitchcock served as a “director’s advisor,” supervising the way the footage was edited. He demanded that the documentary emphasized long shoots and frequent pans, so that people wouldn’t question its authenticity. He was also blown away by the stark contrast between the quotidian (the lives of Germans living near the camps) and the ghastly (the nightmare within), and requested that the film use maps to highlight their proximity, driving home the message that these German citizens knew exactly what was going on.

“Hitchcock himself is reputed to have been very nauseated by the footage,” says Singer. “He saw it in the cutting room and then didn’t want to watch it again.”

Bernstein’s time, however, soon ran out. According to a June 1945 memo from the Psychological Warfare Film department, Bernstein, lost in an artistic quagmire, was relieved of his duties. “Here he was trying to make a propaganda film, and he ended up making a great documentary,” says Singer. “And that’s not really what the government wanted.”

The Americans then installed acclaimed Double Indemnity filmmaker Billy Wilder as supervising director, and used some of Bernstein’s footage to create a short propaganda film entitled Death Mills. Whereas Bernstein’s film was artistic and captured the full breadth of suffering, Wilder’s version is described as a “hectoring short film” that “merely accused the Germans of committing these crimes.” Still, to the Americans’ credit, the film premiered in Germany.

“We must show it to as many Germans as possible,” Wilder says in a 1988 interview. “They will say: it’s a lie. These are extras with makeup. This is Hollywood… made by Jews. But where would we have got them from? They were just skin and bone!”

While Bernstein’s footage was eventually used as evidence against Nazi officials during the Nuremberg trials, his original, uncensored vision never saw the light of day. Singer’s film claims that the British government chose to kill Bernstein’s comprehensive documentary largely for two reasons. First, that Britain didn’t want to “further alienate the German people,” who were in a rebuilding phase and, with hints of the Cold War starting to appear, were seen as a potentially ally against the Soviets. And second, that the British were worried about the budding Zionist movement and weren’t interested in galvanizing support for a Jewish state.

“I don’t think that in government they were concerned about whether the German people were denying it ever happened,” Singer says of the film’s cancellation. “Morally and ethically it should be done, but in practical terms, it was just bad, bad timing. Churchill had just been kicked out of government and was more sympathetic, and there was a new foreign secretary, Ernest Bevin, who was virulently anti-Zionist. I don’t think it was a conspiracy, or that any censorship was going on.”

As for his own documentary Night Will Fall, Singer feels it’s vital to inform audiences of what really happened at the Nazi concentration camps in order for the wounds of the past to heal, but not be forgotten.

“A lot of people are worried about whether we should be showing atrocity footage like this to people, is it meant to shock, and is there a pornography element to it,” says Singer. “I believe the opposite. Put in context, it’s incredibly important.”