CANNES, France (AP) — David Greaves was 26 when his father, the pioneering filmmaker William Greaves, asked him to be one of four cameramen documenting a historic gathering in Harlem.

In August 1972, William Greaves assembled as many artists, writers, poets, musicians and organizers from the Harlem Renaissance as he could. They came for a cocktail party at Duke Ellington’s Harlem townhouse. There, they talked about the seminal 1920s cultural movement: what they remembered, who not to forget, what it all meant.

“My father would say, ‘Capture the life that’s happening,’” David recalls.

It took more than half a century for the result to see the light of day. But 54 years after that gathering, “Once Upon a Time Harlem” screened this week at the Cannes Film Festival.

No movie in Cannes had a longer road to get here. William Greaves died in 2014 having never finished what he felt would be his most enduring work. With David ultimately stepping in as director, his family saw it through.

“It’s not the film he was thinking of in his mind,” David Greaves said in an interview by the beach in Cannes. “But it’s definitely the film he would have wanted.”

It was fitting that “Once Upon a Time in Harlem” got its moment in Cannes. William Greaves’ 1968 opus, “Symbiopsychotaxiplasm: Take One,” was rejected at the time by the festival. The experimental documentary would nevertheless grow to become revered by filmmakers, and in 2015 it was added to the National Film Registry.

Given that history, it was hard for David Greaves to summarize what it felt like to be at the festival, bringing his father’s work finally to cinema's global stage.

“It feels magical,” he said, his eyes welling up. “Even surreal.”

Now, “Once Upon a Time in Harlem” might be the nonfiction movie event of the year. Following its premiere earlier this year, Neon acquired it and is planning an awards campaign. It will play at top fall festivals. After seeing an unfinished cut of the film last year, The New Yorker’s Richard Brody called it “a film for the ages.”

Gathered that day in Harlem was a spectrum of Harlem Renaissance luminaries including the poet and novelist Arna Bontemps; the artist Romare Bearden; the actor Leigh Whipper, then 96; Ida Mae Cullen, the widow of the poet Countee Cullen; the musician Eubie Blake, the poet and painter Richard Bruce Nugent; the scholar John Henrik Clarke.

Together, they take turns reminiscing about the flourishing in Harlem — laughing, arguing over and celebrating their place in Black history. In the 1970s, it wasn’t as widely recognized. Now, the film arrives at a time when African American history is increasingly under siege in America.

For David Greaves, the definition of the Harlem Renaissance is simple: “It’s the wellspring.”

“People say: How can there be a renaissance? People without history arriving here?” he says. “I first wanted to open the film with a history stretching back to Africa. Everyone was like, ‘OK, OK, where’s the party?”

Instead, the documentary opens with a poem that Greaves felt expressed it all: Langston Hughes’ “The Negro Speaks of Rivers.”

William Greaves’ original purpose with the footage was to use it for the 1974 film “From These Roots.” But he instead opted to use archival photographs. Over the years, he would return to the 1972 footage in Harlem but never shaped it into a film.

After he died in 2014 at the age of 87, his widow, Louise Archambault Greave, took up the project. She died in 2023 but not before securing funding for the restoration.

“Louise was a lock protecting the footage. She told the Smithsonian, who asked for a copy, ‘No!’” David Greaves says, laughing.

Though he was raised assisting on his father’s films, David Greaves didn’t remain in moviemaking. He co-founded and ran the progressive Brooklyn community newspaper Our Time Press. It was years before he stepped forward to direct. His daughter, Liani, is a producer.

“Louis was talking about directors. ‘Who could we get?’ I just sat there and said, ‘I don’t know,’” David Greaves says. “Then it came to a point in the editing room after she had passed, (adviser) Marcia Smith said, ‘Who’s going to direct this? Are you going to direct it?’ And I said, ‘Yes.’ I couldn’t imagine anyone else directing this film. I just couldn’t do it.

David Greaves barely remembers what he shot in 1972. He's seen fleetingly in a mirror at times. But it was too long ago to really remember — longer than the time span from the Harlem Renaissance to that townhouse meeting. “Once Upon a Time in Harlem” is a luminous artifact of the past, twice over.

“Usually after seeing a movie, people say ‘Congratulations,’” says Greaves. “Here they say, ‘Thank you.’”

Greaves can hardly get the words out before the tears come streaming again. He wipes them away, lifts his head and smiles.