Many stories have been told about the Kennedys, but very few have come from the family’s perspective. One of those rare exceptions will be seen this Friday at Guild Hall when Rory Kennedy’s documentary, “Ethel,” is shown as the fifth and concluding film in the Hamptons International Film Festival’s “Summer Doc” series.

Ethel Kennedy has deliberately stayed out of the public eye. She has not granted an interview in at least two decades, which in itself makes this documentary, which will be seen on HBO this fall, unique.

The film combines family and historical footage with interviews with Ms. Kennedy and seven of her 11 children to portray an especially strong woman who raised her family alone after the June 1968 assassination of her husband, Senator Robert Kennedy. Rory Kennedy, the couple’s 11th child, was born six months after her father’s death.

Ever since her film, “American Hollow,” a documentary about an Appalachian family, made its debut at the Sundance Film Festival in 1999, Ms. Kennedy has been a prolific producer and director. An Emmy Award for “Ghosts of Abu Ghraib” is among the honors she has earned. Her last full-length documentary was “The Fence (La Barda),” which opened Sundance in 2010 and was shown on HBO later that year.

One would think that if there were to be a film on Ethel Kennedy, her filmmaker daughter would be first in line to do it. But Ms. Kennedy wasn’t all that eager when the idea was first floated to her by Sheila Nevins, the president of HBO Documentary Films, especially given Ethel Kennedy’s rejection of all interview requests over the years.

“I thought she would have said no even when I asked her,” Ms. Kennedy said during a telephone interview last week. “I was kind of banking on it, frankly. Initially, I did resist the idea, but Sheila kept insisting. Finally, I said, ‘Okay, I’ll ask my mother, but I know what she’ll say.’ And she surprised me by agreeing to do the film. I think what changed for her was several of my siblings had been talking to her about doing a book that would include all the amazing stories she has, and I come along and propose a documentary.”

The result, according to HBO, is “a treasure trove of such revealing and little-known anecdotes, recounted by those who know Ethel best: her family. Essentially a decades-long love story between Ethel and her late husband, it’s the first time a film has been made about the Kennedys from inside the family and contains the first extended interview Ethel has done in more than 20 years.”

The film explores the political awakening of the former Ethel Skakel, whose husband, Robert, became Attorney General after his brother, John, was elected President in 1960. In that role, he was on the front lines of investigations into organized crime and the Civil Rights movement.

After his brother’s assassination, Robert was elected to one of the U.S. Senate seats in New York. His own killing came as he was running for the Democratic nomination for President, on the night he won the California primary.

Though no other filmmaker could have the access to interview subjects and archival records as Ms. Kennedy did, she was still apprehensive about taking on a project with many personal reverberations.

“I didn’t know if I could handle the subject matter or not,” she admitted. “It was certainly daunting. But I’d reached a point in my career when I’d made enough films that I had enough confidence in my work and in my ability to shape this story and take it out into the world the way it deserves. And, if I was going to be able to do it, now was the time.”

She explained that the film weaves her mother’s story and what was going on in her family at a personal level with what was happening in the country in the 1950s and ’60s.

“We were all experiencing that together,” she said. “It’s not that my mother was always on the front lines but she was certainly present during many significant events. The Cuban Missile Crisis is a good example. Most of us know what the country was going through, but what was the family going through behind the scenes? The Jimmy Hoffa hearings and the threats to my family during them is another example.”

She emphasized that the documentary is far from a history lesson.

“My mother also has a rascally side to her too, and she’s a woman with a lot of moxie,” said Ms. Kennedy, whose film company is called Moxie Firecracker Productions. “It was important to portray that too, and there is a lot of humor in the film. My mother liked to have fun. At one party, she pushed a member of President Kennedy’s cabinet into the family swimming pool.”