Forget Guillermo del Toro’s Frankenstein. The monster movie of the year is a historical documentary. Like Mary Shelley’s gothic classic, Andres Veiel’s Riefenstahl is a cautionary tale about the moral responsibility of creators, a parable about what happens when ambition is uncoupled from ethics. It is also a film about beauty and ugliness, and how those who assign moral value to the former over the latter will come to be haunted by their choice.

With films such as Triumph of the Will and Olympia, Helene “Leni” Riefenstahl shaped the cinematic language of Hollywood, advertising and sports broadcasting, pioneering the swooping crowd shot and the sculpture-like stylisation of athletic bodies. That she had a fascination with Hitler, and that Hitler was fascinated by her, is well known – she herself never disputed it.

She did, however, insist that her films were essentially apolitical, telling interviewers after the war that her unquenchable ambition was such that she would have just as happily made films for Roosevelt or Stalin.

Riefenstahl is produced by Sandra Maischberger, a high-profile German TV journalist who in 2002 was one of the last people to interview the director, a year before she died aged 101, and knew that she had had hoarded an enormous archive, including diaries, letters, manuscripts for memoirs and tapes of phone conversations with friends and foes.

Diving into these 7,000 unopened boxes gives Riefenstahl a journalistic grounding – it is here that the film unearths material that puts a lie to the aestheticised version of her work that she tried to present after the war, suggesting she was a direct witness to war crimes she claims to have known nothing about.

The horror, though, lies in the telling. Almost a quarter of a century after he made one of the best films about the legacy of the Baader-Meinhof terror group, Black Box BRD, (and eight years after a slightly too reverential portrait of the artist Joseph Beuys) Veiel again demonstrates that he is a master at using dead air and white noise for dramatic effect.

The most chilling moments in this psychogram are not some revelatory document unearthed from the archive, but Riefenstahl’s own silence when confronted with her complicity: a chair inside a French TV studio that is left empty after she pulls out of an interview at the last minute, the crackle of a microphone she tears off in a rage mid-sentence.

Narration is minimal; Riefenstahl incriminates herself with her own words, and a few deft touches in the editing room. The message of Triumph of the Will, we hear her insist, was never a political one, it was only about the need for peace. Then a cut, and a scene from the film itself: Der Stürmer publisher Julius Streicher on a podium, telling a hall full of ecstatic Nazis that the German people must protect the purity of their race or they will perish.

“Politics”, Riefenstahl tells an interviewer at one point in the film, was “the opposite of what has fulfilled and fascinated me all of my life: art”. But fascism is not politics: it is the suspension of political processes in favour of a spectacle that promises instant gratification of the public’s darkest desires. Veiel seems to concur with Walter Benjamin that what Riefenstahl was doing with her films – aestheticising politics into a fetish of physical perfection – was precisely what fascism is.

His film ends with a scene that is both mundane and one of sheer horror. An ageing Riefenstahl preens herself in a mirror before going on camera; a woman whose films allowed only for beauty desperately searching for the taut facial features of her youth. As she directs the makeup artist to disappear the lines on her face, there is no flicker of self-awareness, only vanity and unrelenting self-belief.