Since his German wasn't so good, he asked an elderly man sitting next to him to translate the film, which tells the story of Shlomo Perel, a Jew who survived the Holocaust by falsifying his identity — and who served in the German army for part of World War II.
After the movie, the man told Rigg that his story was similar to Perel's. Over a drink, the man told Rigg about his experiences as a "quarter-Jew" who had served for Germany on the Russian front.
The conversation fascinated Rigg and spurred him to investigate whether there were more soldiers of Jewish descent in the Nazi army.
He began checking — and sure enough, there were.
What's more, little scholarly work had been done on these mischlinge, as the Nazis called Germans with Jewish roots.
"They suffered the same fate in academic life that they did in the Third Reich. Nobody wanted them. Nobody claimed them. So nobody knows about them," Rigg, 31, said in a recent interview.
The encounter launched a 10-year odyssey for Rigg that culminated in "Hitler's Jewish Soldiers," which is making waves in both the media and academia.
The Chronicle of Higher Education printed a lengthy article on Rigg and his book, and he is slated to be the subject of a segment on NBC's "Dateline" on Sunday.
In the book, Rigg tells the strange-but-true story of these wartime German soldiers with Jewish roots. Based on interviews with more than 400 of these former soldiers, along with some statistical extrapolation, Rigg concluded that more than 100,000 such soldiers — who were considered Jewish, according to Nazi racial laws — served in the German military.
Many researchers consider that number an exaggeration and dismiss Rigg, who teaches at the online American Military University, as publicity-hungry.
"This is not a bombshell," Raul Hilberg, one of the deans of Holocaust scholarship, recently told The Chronicle of Higher Education. "We have known that there were thousands" of men with Jewish roots "in the German army."
Some also have taken aim at the book's title. After all, Rigg himself says that only 60 percent of the "half-Jews" and only 30 percent of the "quarter-Jews" who served as soldiers were Jewish according to Jewish law.
Many didn't even know they were Jewish because their families had assimilated.
But many scholars say Rigg's book, based on his doctoral dissertation at Cambridge University in England, casts new light on Nazi policy and the Holocaust.
Rigg's "diligent" and "sustained" research calls into question some previous assumptions about Nazi policy during the Holocaust, according to scholar Michael Berenbaum. author of "The World Must Know: The History of the Holocaust."
Rigg's book "shows that there was a greater degree of flexibility in the anti-Jewish policy than previously realized," he said.
Berenbaum cited Rigg's evidence showing that, as late as 1943, Hitler was spending his time pondering the fate of individual soldiers with Jewish roots.
While the German war machine was focused on battling allied forces, Hitler was "deciding whether this guy's face is Jewish. It's unbelievable," Berenbaum said.
Rigg admits that it's a bit unbelievable that he became a Holocaust scholar.
"Ten years ago, if you had asked me that this was going to happen, that we'd be sitting here talking about this, I'd be like, 'No way.'"
Tall, fit and square-jawed — and prone to use the words "honored" and "gentlemanly" in conversation — Rigg looks more like a former football player and Marine from Texas — which, in fact, he is.
As a teenager, Rigg attended the Fort Worth Christian Academy and spent time on Protestant missions.
While researching his family history that summer in 1992, Rigg found some records indicating that many of his mother's ancestors were Jewish.
"I have some ancestors who were running around in skirts in northern Scotland hacking up each other. That's part of my tradition as well. I also have some tradition going to the Temple Mount," he said.
But it wasn't just his Christian upbringing that made him an unlikely candidate for research into the Holocaust. Throughout his research, he spoke to many scholars who dissuaded him from his work.
He was told that the subject matter was either too tangential or would cause problems for Jews, Rigg recalls, but he turned the criticism into a challenge.
It wasn't his first academic obstacle: As a young child, he failed first grade twice. Only when he was placed into a university-affiliated school did he begin to flourish.
After high school, he was rejected from the Ivy League schools he had dreamed of attending. So he spent a fifth year of high school studying and playing football at an East Coast private school and then was accepted at Yale.
Even today, Rigg appears to be motivated by the discouragement he received from some scholars and talks about his time spent in "the bowels of the academic establishment."
At a lecture last month at the Leo Baeck Institute in New York, Rigg said some scholars "exuded an air of academic arrogance that irritated me."
Armed with that motivation, as well as some encouragement from his family and from scholars such as Jonathan Steinberg, his doctoral adviser at Cambridge, Rigg persevered.
After spending time with some of the soldiers, he felt he owed something to them — and to what he calls truth, which he uses without an ounce of irony.
Rigg himself contributed to "personal truth" — "outing" some of those soldiers' Jewish roots to their own families.
Some of the soldiers Rigg became interested in their Judaism after the war, but others died without telling anyone — and Rigg was the one to inform their families.
Even if many of his subjects didn't consider themselves Jewish, their experiences during the war highlight a gray spot in the world of the Holocaust, according to Rigg.
"Are they perpetrators or are they victims? Do they share the guilt or do they share the victimhood? They're between two stools all the time."
So is Rigg, in many ways. Raised a fundamentalist Protestant, he studied at the Ohr Sameach Yeshiva in Jerusalem while conducting his research and says he now professes no specific religion beyond general "tolerance."
His time at a yeshiva was just one of the turns Rigg's life has taken during the last decade. He also spent time in a program the Israeli army runs for volunteers from abroad, and even did a stint in the U.S. Marine Corps from 1999 to 2001.
But, he says, he made a commitment to his subjects to tell their story. He has done that both through his book and through an archive in the German city of Freiburg , which he has filled with the fruits of his research.
"Now I've honored that commitment and I can walk away after all this is done, and be happy," he said.