NEW YORK CITY — When he was only a boy, filmmaker Lloyd Handwerker, 57, picked up hints that his grandfather Nathan — of Nathan's Famous frankfurters — was a remarkable man.

As a child when he walked into the now-national chain's original store at Coney Island with his grandfather, customers and staff would stop for a moment to stare.

In school, his peers would ask if he was related to "The" Nathan Handwerker and then recount stories of their own frankfurter memories involving the legendary store, which opened in 1916 — and has since grown to sell about 450 million hot dogs worldwide.

But the behind-the-scenes history of the brand, from Nathan Handwerker's journey as a Polish teenager who immigrated to the US in 1912 to becoming one of America's most well-known hot dog vendors to growing tensions within the family, had never been told.

That is, until Lloyd Handwerker spent the past 30 years documenting it, with the end result becoming "Famous Nathan," a feature-length documentary that premiered at the Tribeca Film Festival last week.

"I just thought, 'These stories are going to disappear and I have all these elderly relatives who worked there, who knew my grandfather, it's time to get something down on tape," said Handwerker, who began interviewing subjects for the documentary in 1984. He grandfather passed away in 1974 when Handwerker was 17.

Throughout three decades, Handwerker trolled eBay for snippets of Nathan's Famous footage, advertised in Jewish newspapers for content and collected the photos of former hotdog servers, all in the hopes of knitting together an honest story of his family's patriarch.

The search for content unearthed documentary gold, such as four hours of audio interviews with Handwerker's grandfather recorded by a cousin.

"I was ecstatic to be able to hear him again, to hear his voice and his stories," he said, adding that three quarters of the recordings were his grandfather's retelling of his Polish childhood. Handwerker painstakingly chose just 16 minutes of the recordings for his film, a first-person narration that acts at the documentary's backbone.

Handwerker's own father also kept a 1957 in-house PR film on the business in a safe for half a century. The footage gave the documentary its only video interview with his grandfather.

The final effect with the natural grain structure of footage from so many different eras, gives the documentary the nostalgic feel of a giant Instagram video flicking between various filters and treatments.

The documentary paints the picture of an immigrant seizing the American dream, with the elder Handwerker solidifying his toehold in the franks trade by undercutting a neighboring frankfurter's price by half, charging five cents a frankfurt.

But Handwerker didn't ignore the tension that leaked out from a family at the helm of a multimillion-dollar business. The family sold Nathan's Famous in 1987 for $17 million.

His own father, Sol Handwerker, left the business in 1963 and began his own hot dog store called Snacktime in Midtown. It stayed open for 15 years.

Now, no one from the family is involved with the running of Nathan's Famous, but as an extended family they still own the building where it all began — 1310 Surf Ave. in Coney Island.

"I didn't try to present my grandfather as a god even though he was in some ways, especially in relation to his restaurant," said Handwerker.