Tim Wardle’s documentary starts as a joyous celebration of three siblings separated at birth finding one another nineteen years later. It gradually morphs, however, into something unnerving and perhaps even horrific, raising troubling issues about scientific experiments that might begin with the best of intentions but can have unintended results, not to mention ethical problems.

The titular subjects are Robert Shafran, Edward Galland and David Kellman, triplets born on July 12, 1961, at the Long Island Jewish Medical Center to an unmarried mother who gave them up for adoption. The well respected agency tasked with placing them was Louise Wise Services, and it separated them, choosing three families of different socio-economic circumstances. They all grew up knowing nothing of the others.

In 1980, Robert began his freshman year at Sullivan County Community College, and arrived on campus to an extraordinarily fulsome reception from students who seemed to recognize him. One of them informed him that he had a double—Eddy, who had attended the school the previous year. Together they drove to Galland’s house, and the two were reunited.

News of the serendipitous reconnection went viral, to use today’s terminology, and it reached David, who looked at their photo and saw himself. He called them, and another reunion occurred. The three brothers were treated as a national phenomenon, appearing on major TV talk shows to emphasize their amazing similarities of personality despite their long time apart, and becoming regulars in the Big Apple’s nightlife. They eventually parlayed their newfound celebrity into a joint business, a SoHo steakhouse. All this is presented through interviews with Shafran and Kellman, as well as family and friends and the Newsday reporter who broke the story, as well as a wide array of archival materials—film clips, news reports and stills, along with a few dramatic recreations.

Of course, their adoptive parents wanted to know more about the reasons why the original process had gone as it had, and visited the offices of the Wise agency for an explanation. They were told that it was difficult to place three infants with a single family (though that option had never been offered to any of them), and it was considered more appropriate to separate the boys. One of the fathers, however, had a disquieting moment when he saw the Wise officials celebrating over how the meeting had gone, saying they acted as though they had “dodged a bullet.”

(Warning: spoilers follow. If you want to preserve the surprise the film has in store until viewing, read no further.)

As would eventually become clear, they had good reason to feel that way. As the result of investigation by Lawrence Wright, who was studying the dominance of “nature” versus “nurture” in the lives of twins and triplets for an article he was preparing for the New Yorker and a larger book project and is one of Wardle’s interviewees, this was no isolated incident. The triplets were in fact part of a study in which the Wise agency had cooperated with Dr. Peter Neubauer, a noted New York child psychiatrist, and his team of researchers in separating twins and triplets and then following the development of the individual children over time in different socio-economic circumstances.

Wardle’s film includes long excerpts from interviews with Shafran and Kellman, who are understandably angry at having been used as unknowing guinea pigs, especially after Galland’s life ended tragically. Others who were subjects of Neubauer’s work and were later reunited are similarly upset. At one point Shafran compares Neubauer’s project to what the Nazis had done, and it’s almost inevitable that one will think of Josef Mengele’s use of twins in his notorious death-camp experiments.

But there are other viewpoints represented as well. Wardle talks with two people who served as young research assistants to Neubauer, and while they offer somewhat different perspectives—one is somewhat skittish and apologetic, but the other bluntly dismisses the idea that the study should be judged as anything but well-intentioned and properly conducted by the standards of its time—they play down any sinister motives behind what happened.

Nonetheless one is bound to be dissatisfied with the revelation that Neubauer never published any studies based on his research, and at his death in 2008 he deposited all materials about the experiment at Yale University with the proviso that they not be unsealed until 2066. While some subjects have tried to gain access to the files, they have managed to see only some heavily redacted portions of them, and most of the study remains a mystery, its purpose and results more a matter of conjecture than fact.

Wardle lays out this extraordinary story skillfully, cannily shifting from the good news of reunion to the disquieting revelation of how the original separation occurred to generate not just curiosity but a degree of outrage as well. “Three Identical Strangers” tells a fascinating and unsettling true story, and does so well, without pretending to have all the answers.