More than a flowering of the arts at the dawn of the 20th century, the Harlem Renaissance marked the breakthrough moment when Black Americans asserted their creative control, defining a culture that had been purpose-built to exclude them. The imprints were all over the Jazz Age – on bookshelves and radio, in flapper dresses and cloche hats, in Hubert Julian’s daring flights and the fractious debates between Marcus Garvey and WEB Du Bois.

Like the David committee of the Italian Renaissance or the American Revolution’s Continental Congress, the Harlem Renaissance is one of those moments that makes you wonder: what was it really like? The Dark Tower salons? Those Cotton Club nights? The rent parties with all those New Negroes – a heady brew of southern émigrés, Caribbean vanguards and New York lifers – holding forth and hootin’ and hollerin’, high on the fact that they had found one another? Could they appreciate the significance of the moment as they were making it? “I remember being in Brooklyn in the 1990s and seeing various artists around,” says film-maker Liani Greaves, reflecting on her early career working with Tupac on Above the Rim and the director Hype Williams on Biggie music videos. “Brooklyn Moon Cafe was the spot. Sometimes we forget that it is The Now. We’re constantly looking to the next thing, looking outside.”

Once Upon a Time in Harlem looks back – to the August day in 1972 when Liani’s grandfather, the prolific and pioneering film-maker William Greaves, threw a cocktail party at Duke Ellington House, invited over the surviving luminaries of the Harlem Renaissance and switched on the cameras. (With Ellington himself too ill to attend, it fell to his sister, Ruth, to play host.) For four hours, these extraordinary partygoers – some seeing one another for the first time in 50 years – reminisced, made merry and reckoned with the Harlem Renaissance’s modern-day legacy. The 100-minute film itself is a time capsule, a cinematic Hall of Mirrors at Versailles in which the Harlem Renaissance gazes back at its own legacy, leaving future generations to unearth, interrogate and work from the reflections they find.

It’s the kind of artefact today’s Black creatives should have in their library – for inspiration, for accountability, to ground themselves. But until hard copies are available – “That will exist!” says William’s son, director David Greaves, adding that streaming is still a long way off – audiences will have to catch Once Upon a Time in theaters, as the film opens nationwide this month. “We’ve always imagined people coming in groups, going with their family,” says Liani, a producer on the film. “We want people to be in community, visit with the film, and really experience it in that way.”

Following in the tradition of A Great Day in Harlem, the documentary about Art Kane’s stunning photograph of jazz greats gathered on Harlem stoop, Once Upon a Time is a home movie in every sense. William Greaves – an Actors Studio member alongside Marlon Brando before directing the groundbreaking meta-documentary Symbiopsychotaxiplasm – devoted himself to the project until his death in 2014 at age 87. Greaves kept rolling after the party, through the 80s and 90s. Liani remembers William and his wife, Louise Archambault Greaves, turning up with their cameras to a Harlem Aids walk she was working in the 2000s. Among Liani and David’s inheritances was some 60,000 feet of 16mm film held in storage. “I think they were shooting a lot of events, looking for evidence of the Harlem Renaissance,” Liani says. “But it felt to me that they had kind of lost their way. And just like in Symbiopsychotaxiplasm, he had to kind of step away in order for the film to be great.”

So Liani and David kept the focus squarely on the party. They enlisted the help of preservation specialist Bill Brand to restore the 1972 footage, an 18-month process, before sending it off to a finishing house. “They did stuff that we could never have done 50 years ago,” David says. “Color correction, cleaning and also – I’ll be honest – taking hair out of the gate.” The results are visible in the richness, warmth and texture of the footage – the kind of craftsmanship movie-making used to be about.

And then the titans of the Harlem Renaissance start filling the frame: the photographer James Van Der Zee, demurring from an invitation to play Duke’s piano before letting his shockingly nimble fingers wander. Ragtime composer Eubie Blake reeling back to the time the papers misquoted him saying he’d never extend his hand to a white man. (He said he’d never extend it first, a distinction that mattered under segregation.) Regina Andrews, the essential librarian and curator, considered whether a Black artist could have worked in isolation while the movement was happening around them. “There was no way not to be caught up,” she says. “You were pulled along.”

As the 1972 reunion unfolds, the film scrambles to capture every moment, following the connections and conversations across three rooms. David was part of the six-man crew his father assembled for the shoot, each member having to split their attention between the scene unfolding before them and whatever might be bubbling up somewhere else. There’s a lovely moment when one cameraman catches William slating him on from the edge of a scene, the silent soundman confirming with his eyes as another cameraman crosses the shot. (Syncing the picture to the sound was another puzzle that came together in restoration.)

As William strains to impose a bit of film-making order on action already beyond his control, it’s clear he wouldn’t have it any other way, that he’s getting exactly what he wants from this crowd – big personalities, lively crosstalk, a tangible sense of those bygone salons. Partygoers in their 70s and 80s, flashing their modern finery, are stirred to youthful passion when the subjects that once animated them resurface – especially in the one-on-one interviews William Greaves shot with some Renaissance contributors after the party. “My father had them wear the same clothes so they would not be clashing,” David remembers fondly.

It’s hard not to be awestruck when the Negro Actors Guild president, Leigh Whipper, drops into his Haile Selassie monologue from the Stalinist film Mission to Moscow, or when the civil rights activist Richard B Moore delivers a perfect recitation of the Claude McKay’s poem If We Must Die on cue. “I always say this is what it looks like when your brain is not on electronics,” Liani says. “I have to imagine many of them were writing with one another, to one another – some by hand, which is a different kind of brain skill. They were brilliant people.”

The Harlem romantics also had a knack for expressing their disagreements with one another in erudite style – polite shade, we call it now. In one meaty scene, the multimedia artist Romare Bearden takes exception to the way artists like him were minimized in the seminal book on the Harlem Renaissance by Nathan Huggins; the Harvard professor is standing across the room, holding it. Huggins, though, does offer a credible defense – arguing that art and music are intrinsically more difficult to translate than the written word. “It is kind of poetic, the way they clap back,” Liani says. “It’s all in love and friendship.”

Still, there wasn’t genuine affection for everyone in the room. Apparently, the presence of George Schuyler – the commentator who came up during the Renaissance before turning against progressivism and, later, the civil rights movement – was met with twisted faces and cold shoulders. One barb directed at him was cut out entirely. It speaks to the depth of the disagreements among the Renaissance’s vanguards, and the intensity with which they still cared. “It’s like [Garveyite community organizer] Thomas Harvey said at the end of the film: ‘We were on different roads, but all going toward the same goal,’” David says. “It shows that the Renaissance was not a monolith of thinking.”

But the issue that really hung over the party was how the Harlem Renaissance would be remembered. Already, William’s guests were lamenting how the younger generation had moved on to zeitgeistier fare: comic books, funk music, blaxploitation films. David Greaves makes a point of tracking the change his father set out to chronicle in the documentary, moving from James Van Der Zee’s gorgeous black-and-white photographs of the neighborhood in full flower to William’s 70s footage of vacant faces and graffiti-strewn walls.

Now those kids are the ones clamoring for respect for their artistic contributions. It shows how quickly times change, and how much Black creatives stand to gain by looking to the past as they take the culture forward. “I often think about this film as a recipe for a renaissance,” Liani says. “If you watch, the [Black Renaissance vanguards] tell you what to do. They talk about the importance of being in community, of being audacious. That’s how you create a moment.“

• Once Upon a Time in Harlem is out in US cinemas now and in the UK and Australia at a later date