Almost every new undergraduate experiences a mixture of excitement and trepidation when faced with the prospect of trying to work out where he or she fits in after leaving home for university.

Imagine, then, Robert Shafran’s reaction when he arrived at Sullivan County Community College, in 1980, aged 19 and alone. Instead of being just another new face, was greeted like a returning hero. He’d not been especially popular in high school but, now, people he’d never met before were showering him with hugs and kisses. Weirdly, he recalls exuberantly to camera at the start of Tim Wardle’s endlessly fascinating and gripping documentary, Three Identical Strangers, they also kept calling him Eddy.

Robert knew he was adopted. What he was about to discover, though, was that he had a twin brother, Eddy Galland, living in Long Island. After their heart-warming reunion was picked up by the press, the story — in the words of a journalist who helped break it — went from “amazing to incredible” when David Kellman saw the twins’ photograph in a newspaper, and recognised himself in their faces and the way they held their fleshy hands.

Born July 12, 1961, the triplets had, at six months old, been placed with families from different social classes by Louise Wise Services in New York. When their kinship was discovered, their adoptive parents made a direct attempt to find out why no one had mentioned their sons had siblings, but were stonewalled. They sought legal help; however, Louise Wise Services had the monopoly on Jewish children for adoption on the East Coast, says Wardle, and when the families spoke to lawyers, “none of the law firms would take [the case] on because they were like, ‘we’ve got partners who are trying to adopt from that agency.’”

It wasn’t until the early 1990s, when Robert unexpectedly received a phone call from Lawrence Wright, a journalist researching a story for the New Yorker about identical twins reared apart, that the truth started to emerge, and things, as one of the brothers’ friends says, “got funky”. At this point, the documentary abruptly changes from fizzy, first-hand recollections about reunions, media storms, and living the fast-life in 1980s New York, to a darker tale of unethical scientific practices and institutional overreach.

“If you were talking about this in genre terms,” says Wardle, “it switches from being almost like a John Hughes High-School movie, to a kind of identity thriller.”

The less you know about Three Identical Strangers before watching it, the more powerful and mind-boggling the experience. Suffice it to say that the reaction from Jewish audiences to the involvement of Jews in what Robert bitterly calls the “Nazi sh*t”, to which he and his brothers were subjected, has been shock and dismay — one suspects not least because of the role played by a leading Jewish child psychologist called Peter Neubauer.

Wardle says his wife, who is Jewish (her family loaned the director a Yiddish dictionary to translate some of the phrases people were using), finds the documentary “really hard to watch because of the legacy of the Holocaust, because of Mengele,” and because she feels what was done by Jewish scientists was “completely abhorrent and just wrong on every level.”

He notes that a central irony in the film is that David’s adoptive family and Neubauer’s family were Austrians who fled the Holocaust and later found themselves on opposite sides of an ethical and moral divide.

“It was important for me to acknowledge those parallels and that that history is very troubling for Jewish people in the film,” says Wardle.

He stresses that, for him, though, it was always a universal story. Indeed, the profound questions the film raises about how we become who we are, the importance of family, and the issue of nature-versus-nurture, are relevant to everyone equally, regardless of creed, colour or race.

Wardle jumped at the story when producer Grace Hughes-Hallett brought it to him at Raw, where he was running the development team. Few ideas these days feel new, he says, but this one definitely was.

“It belongs to that ever diminishing group of stories that exist in the pre-internet era where the people are still alive but it hasn’t been done to death and recycled a million times by the internet.

“And it made it a real challenge making the film, because you can’t just Google stuff about this. There was no definitive book or anything like that. We properly had to go out and do old-school journalism.”

He was amazed that no one had told the story before. Then he found out that there had been three attempts by major US networks in the ’80s and ’90s, which had all been cancelled suddenly by “people higher up in the network, higher up the food chain.

“We spoke to a triple Pulitzer-winning journalist working for the New York Times who, in the ’90s, made a documentary and had it basically ready to go and it was pulled by the network. He was never given an answer as to why that happened, so there were a lot of conspiracy theories.”

Lawrence Wright, a consultant on Three Identical Strangers, says in the film there are a lot of powerful people who would like silence around what happened. Wardle agrees. “It made it difficult because we were completely paranoid the whole time.” This was made worse by the number of people who said they would talk to him and then went “completely dark”. “So we did start to wonder if people were being pressured.”

Two of the brothers wanted their story to be told, and to discuss what they and others had been put through, but it still took Wardle four years to persuade them to take part. “They didn’t trust anyone to tell it,” he explains. “There was a sense of burning injustice about what had happened [to them] and I think they have been let down by a lot of people.” The process of getting them involved was so “tortuous” that Wardle wasn’t sure they would even turn up for their interviews on the first day. When they did, they gave him all they had.

“I was upfront with them that, if they were going to do it, we were going to be honest and go to some dark places and talk about everything that happened in their lives. There was a nervousness around that . . . [because] what you’re looking for is emotional honesty, and very few people are prepared to go there when the chips are down, and the brothers really did.”

Part of what makes the interviews in the film so compelling is the way that everyone taps back into their original emotions, so everything feels fresh and immediate, almost as though the events they’re describing happened yesterday.

They didn’t, of course, and the brothers aren’t the people they were when they first met each other and “fell in love”. Their relationship broke down due to events described in the film, but when Three Identical Strangers premiered at the Sundance Film Festival in January, they said the process of making it had helped bring them together again and their relationship was now “a work in progress”.

“I wasn’t expecting that,” says Wardle.

He also probably wasn’t expecting the film to become one of the highest-grossing British documentaries ever in the US, where it has just been added to the Oscar documentaries longlist.

Its success, Wardle believes, goes back to his point about universality: “It’s because it doesn’t feel like a niche story, a specifically Jewish story, or a specifically science-y story,” he says.

“It’s about all of us.”

‘Three Identical Strangers’ is in the UKJFF on November 17, and opens in cinemas on November 30