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The HBO documentary “Night Will Fall” is a movie about the Holocaust, a movie about remembering the Holocaust and primarily, at least in formal terms, a movie about a movie. It may not do full justice to all these subjects in its tight 78 minutes, but it’s not a film you’re likely to forget.

The most wrenching sequences in “Night Will Fall” are the scenes it incorporates from “German Concentration Camps Factual Survey,” a movie begun under the auspices of the British government in 1945. Using film shot by Allied cameramen at camps including Auschwitz, Bergen-Belsen and Dachau, and assembled by a team that included Alfred Hitchcock as a supervising director, “Factual Survey” was meant to be a historical document and a teaching tool; among the stated goals of the filmmakers was that it be shown to Germans to prove to them that the horrors of the camps were real.

Amid the rapidly shifting politics of postwar Europe, however, with America and Britain confronting Soviet expansionism and suddenly interested in raising German morale, work on “Factual Survey” was abruptly halted. The finished reels, storyboards and scripts would sit in British archives for more than 60 years, until the Imperial War Museums began a restoration project in 2010.

“Night Will Fall,” directed by André Singer (and making its television premiere on HBO on Monday), tells the story of “Factual Survey,” incorporating archival and current interviews with people involved in its making. It also fills out the story of how the British, American and Soviet cameramen documented the unbelievable scenes that the liberating troops found, and includes touching sequences in which soldiers and camp inmates who appear in the old footage describe their horrific experiences seven decades ago.

And most harrowingly, it incorporates about 12 minutes of the restored “Factual Survey.” Belying its bland, clinical title, the original film, in these excerpts, is a measured but unflinching account, with brutally explicit footage of naked, emaciated corpses lying in stacks, littering fields and being thrown and shoveled into mass graves. Nearly as hard to bear are the scenes (backed by a pointed narration, newly recorded by the actor Jasper Britton) of warehoused eyeglasses, teeth and bales of human hair.

While “Factual Survey” was shelved, the footage it drew from has been used in other films and television programs over the years. But “Night Will Fall” proves that the images never lose their capacity to shock. (The title is taken from a line of narration in the older film: “Unless the world learns the lesson these pictures teach, night will fall.”)

What the new film accomplishes, more than anything else, is to make you wish you could see the original. That’s possible, but not easy. Marking International Holocaust Remembrance Day on Tuesday, and the 70th anniversary of the liberation of the camps, “German Concentration Camps Factual Survey” is being screened at the Museum of Tolerance in Los Angeles on Tuesday and the Holocaust Memorial Museum in San Antonio on Thursday. There are no announced plans to show it more widely, but perhaps “Night Will Fall” will change that.