In 1971 Jerry Lewis, America’s most famous comedian, decided to swing for the fences and make his masterpiece. The Day the Clown Cried was a Holocaust tale about Helmut Doork, a hapless party entertainer who becomes a death camp pied piper. Lewis starred and directed, overseeing every aspect of a fraught shoot in Sweden, but the man misstepped badly and the film never saw the light of day. It has since become legend, buried forever and apparently for good reason.

Now along comes Michael Lurie and Eric Friedler’s flawed but engrossing documentary to pick over the wreckage, shine a UV lamp on the crime scene and – best of all – reveal extended segments of a picture that is destined to remain incomplete. For much of its running time, From Darkness to Light is a jerry-rigged cuttings job, lifting talking-head interviews from Ferne Pearlstein’s 2016 documentary The Last Laugh and folding them alongside fresher insights from the likes of Martin Scorsese and Harry Shearer, one of the few living souls to have actually seen the rough cut. The star attraction here, however, is Lewis himself, talking freely to Friedler shortly before his death in 2017. The Day the Clown Cried, he says, was almost wonderful, almost perfect, which is another way of saying that it was an absolute catastrophe.

Any comic will tell you that disaster is always more fun than success. The bigger the better; the face-plant is a banker. But Lewis’s experience may just be the exception, because there was nothing remotely funny about The Day the Clown Cried, which sounds utterly grisly from start to finish, from its nightmarish production (perhaps not really Lewis’s fault) to the bungled execution (for which he takes full responsibility). One pundit claims that the film erred in tackling the Holocaust instead of a wider, more generalised Nazism, citing the success of Mel Brooks’s The Producers. Another suggests that it may have been ahead of its time, pointing out that Roberto Benigni would later win an Oscar for the similarly-themed Life is Beautiful. Lewis, for his part, isn’t prepared to venture down these rabbit-holes. “It was bad work,” he says sourly. Even decades later, the man still feels the sting.

The Day the Clown Cried was championed by Jean-Luc Godard (who hailed Lewis as a genius) and supported by Ingmar Bergman, who arranged for Harriet Andersson to play the role of Helmut’s wife. The issue, however, was that the film’s approach was all wrong. Or as Shearer puts it: “It’s not a comedy. It’s a serious piece of work. That’s the problem.” If Lewis had been a different kind of performer with a different fanbase (Paul Newman, say, or Donald Sutherland), he might have been able to make it fly. Alternatively, if he’d simply stuck to his established helium-voiced comic antics, he might at least have come up with something bold and dangerous. But he longed to make a sober, serious film in the worst possible way - and in the end that’s what he did.

We like to imagine that the greatest film follies are extravaganzas of bad taste, too wild and crazy for public consumption. The truth here, I suspect, is less exciting because, on the evidence of the clips the film-makers have unearthed and restored, The Day the Clown Cried is just awful. It’s leaden and lugubrious. The actors speak like automatons. We see Helmut in the yard, playfully snagging his nose on barbed wire to the irritation of the guards. “When you’re ruled by fear, laughter is the most frightening sound in the world,” declares one fellow inmate, presumably referring to the listless titters of the Jewish children who have gathered to watch on the other side of the fence. It is these children who Helmut will eventually lead to the gas chamber.

Lurie and Friedler’s handling of the material sometimes feels perfunctory, but the tale they tell is purely fascinating. Lewis looks so stricken by the ordeal that he risks confusing himself with Helmut. It’s as if, in failing to complete and release his film, he was somehow doomed to keep it with him forever, replaying the script’s darkest moments as though they had really happened. “There’s not a day in my life when I don’t think about it,” he says. “I remember walking 65 children into the oven. It was hard, very hard.” Horribly misconceived and appallingly handled, The Day the Clown Cried convinced none of the handful of people who saw it. But it cast a spell on its creator and would haunt him to the grave.

• From Darkness to Light screened at the Venice film festival.