Gloria Vanderbilt relaxing with her family — husband Wyatt Cooper and sons Anderson (left) and Carter — in her NYC apartment in the 1970s. (Jack Robinson/The Condé Nast Publications/Courtesy HBO)

A few weeks before her 92nd birthday this year, Gloria Vanderbilt got up before dawn in New York and took a car to an airport and boarded a private jet belonging to HBO in order to appear at a press conference out west for a new documentary about her and her son Anderson Cooper, the 48-year-old CNN anchor. (The film, “Nothing Left Unsaid: Gloria Vanderbilt and Anderson Cooper,” airs Saturday.) A few hours later, Vanderbilt and a group of HBO execs would fly right back to New York.

“Can you believe it?” she asked, sitting on a hotel sofa on a sunny California afternoon. She was clad in a shiny, saffron-orange tunic and black pants. Her smile was wide and her vowels were stretched. Her hair was coiffed so that it flipped up on the ends, reminiscent of the way she wore it in the late 1970s when she was the queen of her own designer jeans label. “I had never been on a private jet before.”

What? Come on, lady — you are Gloria Vanderbilt. Don’t tell us you’ve never flown private.

“Truly,” she said. “It changes your whole point of view about travel. No schlepping through airports, no taking off your shoes.”

Well, then, you should do it more often.

“Oh, believe me, I absolutely agree with you.”

Later, her son, by phone from New York, explained how it’s entirely possible to descend from Cornelius Vanderbilt’s astounding 19th-century fortune and yet have some of the modern signs of luxury pass you by. Contradictions abound; one really could have dated Howard Hughes 70-some years ago and still be wowed by a trip on a private plane.

Anderson Cooper and his mother Gloria Vanderbilt, 92, walked the red carpet at the New York premiere of the HBO documentary "Nothing Left Unsaid" on Monday, April 4. (  / Reuters)

“Her life is not what most people would think,” Cooper said. “I’ve always felt very protective of her. . . . We’re the only family [each other has] left, more or less. I had a lot of anxiety early in my career about people knowing who my mother was or making assumptions about trust funds or my need to even have a job — which would be false assumptions, frankly. Some people knew a lot about my mom, and some people didn’t.”

Gloria Vanderbilt with her third husband, film director Sidney Lumet, at their 1956 wedding. (Anthony Camerano/AP)

Gloria Vanderbilt and her mother, Gloria Morgan Vanderbilt, in 1935. (Keystone-France/Gamma-Keystone via Getty Images)

Cooper himself grew up largely unaware of his mother’s juiciest stories — and was only vaguely aware that as a 9-year-old she was at the center of an intensely watched custody battle between her mother and aunt. It was the height of the Great Depression, and the press dubbed Gloria the “poor little rich girl,” who stood to inherit $3.6 million at a time when many American families were surviving on a few bucks a week. She was hounded by paparazzi. It left a lasting mark.

“I’ve always thought of my mom as coming from a time and place that doesn’t exist anymore, like a vanished world,” Cooper says at the beginning of the film.

There are very few people alive, he notes, who saw what she saw or remember what she remembers. You don’t have to sit with Vanderbilt for very long to be astonished at the vivid way she can access the past.

As a boy, Cooper was once shown a statue near Grand Central Station of his great-great-great-grandfather, who amassed a shipping and railroad empire. For a while after that, he assumed that everyone’s deceased ancestors became statues.

Cooper’s father, Wyatt Cooper, was Gloria’s fourth husband. He died at age 50, when Anderson was 10. A decade later, in July 1988, Anderson’s older brother Carter, then 23, jumped to his death off a 14th-floor terrace in the family’s Manhattan apartment. Gloria was on her knees begging him not to do it.

Both Anderson Cooper and Gloria Vanderbilt have written and spoken about these losses before, in memoirs and on television. But what they had not done, Cooper began to realize, was talk to each other at length about the past, without restriction. What started as a series of emails between mother and son evolved into the documentary, directed by Liz Garbus, whose recent film about Nina Simone was up for an Oscar. Those emails have also been expanded into an equally fascinating book, “The Rainbow Comes and Goes: A Mother and Son on Life, Love and Loss,” which is out this week.

“Like the movie title says, I really didn’t want there to be anything left between us unsaid,” Cooper explained. “I wanted to finally ask her everything I wanted to know.”

And she answered.

Gloria Vanderbilt leaving church in Long Island at the height of a custody fight between her mother and aunt in the 1930s. (Planet News Archive/SSPL via Getty Images)

A family photo of Carter Cooper and Gloria Vanderbilt. Carter Cooper committed suicide in 1988 at the age of 23. (Courtesy of HBO/Courtesy of HBO)

At the news conference in front of roomful of TV critics, Vanderbilt and Cooper came across as a lighthearted pair — she sat onstage with Garbus, while his giant image appeared on screens above, beamed in by satellite. He teased her about her “date,” at 17, with Errol Flynn. He played his part of the buttoned-down newsman whose life involves covering terrorist acts and grilling presidential candidates; she’s always been the arty, optimistic dreamer. He’s cautious about intimate details that he considers private; she’ll gladly reveal almost anything.

They quibbled, playfully, about the extent of her personal archives and belongings, including the paintings and collages she’s produced for decades and still creates in her studio — a situation he compares to a high-end episode of “Hoarders.” (She’s seen that show and begs to differ.)

Vanderbilts or not, what “Nothing Left Unsaid” makes clear is that Cooper and his mother are, on a slightly grander scale, experiencing an essential and often deeply emotional stage between an elderly parent and a middle-aged child: What are we going to do with all this stuff? What memories does it all hold? Keep or toss? Most of all, what does one need to know about a parent, before it’s too late to ask?

“To be honest, I’ve been worrying about my mother’s stuff since I was 10,” Cooper said, in a phone interview a few weeks ago. “I worried a lot as a kid. I was like this little Felix Unger. I knew that we were spending money on storage fees and I hated any waste of money.” Today, many of Vanderbilt’s belongings are in the basement of a former firehouse in Manhattan that Cooper bought in 2010 and renovated as his residence.

“It’s up to me to go through it all,” Cooper said. One box will produce a trove of letters or other pieces of Vanderbilt family history that will eventually wind up in archives. Another box might contain “a coffeemaker from 1964,” he said, “or cornflakes from 1953 that just got packed up in a move and hadn’t been opened since.”

The movie helped the sorting process. Once HBO got involved and persuaded Garbus to direct, Cooper began sending Garbus boxes of family photos, news clippings, film footage, old videotapes and pieces of his mother’s artwork. Vanderbilt’s paintings inspired the look and impressionistic feel of “Nothing Left Unsaid.” In the swirl of the family custody fight in the 1930s, little Gloria made her own set of paper dolls — an idealized world with a mother and father. In the film, those paper dolls come to life as animation.

Anderson Cooper and Gloria Vanderbilt attend the premiere of "Nothing Left Unsaid" in April. (Charles Sykes/Invision/AP)

In reality, her parents took off on an extended vacation almost as soon as Gloria was born, handing her off to her nurse, a German woman who was nicknamed Dodo. Gloria’s alcoholic father, Reginald Vanderbilt, died when she was 15 months old. Her mother, widowed at 21, moved to Paris, bringing baby Gloria and Nurse Dodo along, even though she lived apart from them.

Gloria’s earliest memory is of lying in a crib and listening to Dodo and her maternal grandmother whisper and gossip about ways to move little Gloria back to New York, to live with her Vanderbilt relatives. Eventually this plan came to pass.

“What’s incredible is thinking about Gloria as a 9-year-old in the heat of this custody [fight],” said Garbus. “She was covered like Lindsay Lohan. There was that level of public interest — the paparazzi were intense. We think about that intense public exposure of children, many people don’t survive that. That’s what makes Gloria so interesting. She’s grown up in palaces, but sometimes that’s just four walls. She’s had this ongoing story of loss and recovery and resilience.”

Near the end of the trial, Gloria’s mother had been outed for having a lesbian relationship, which essentially cost her a victory in the case. (This fact particularly fascinates Cooper, who was in his early 20s when he told his mother he’s gay but did not come out publicly until 2012.) Gloria’s aunt, Gertrude Vanderbilt Whitney, eventually won custody and raised Gloria through most of her adolescence. Not long after a summer trip to visit her mother in California in 1941, teenage Gloria married 32-year-old Pasquale diCicco, a Hollywood agent whose first wife had died mysteriously.

“Wait a minute,” Cooper interrupts his mother as she’s telling the story in the documentary. “You got married to a guy where there were rumors he had killed his former wife?!”

“Sweetheart, I was only 17,” Vanderbilt replies.

DiCicco was abusive, and she left him. Not long after, at 21, she married the legendary conductor Leopold Stokowski, who was more than 40 years her senior. They had two sons, Stanislaus and Christopher. (Stan Stokowski appears in “Nothing Left Unsaid”; Chris Stokowski has not spoken to his mother in decades.) She broke it off with Stokowski in 1955 and married film director Sidney Lumet. When that ended, she married Wyatt Cooper, an author and screenwriter, in 1963.

It’s easy to see how Anderson Cooper, who was born when his mother was 43, feels he’s missed a great deal of her personal history.

In her marriage to Wyatt, Gloria seemed determined to start fresh. She began a fabric-design business, which led to her famous foray into jeans, which led to trendy appearances on talk shows — she even did an episode of “The Love Boat.” She was everywhere on the tube, while at home she had finally achieved an upper-class version of the white-picket fence and simple family life she’d always craved. She was happy to let Carter and Anderson make trips with their father to his home state of Mississippi, where relatives were plenty and the family lore was all about someone else.

“There was so much that had happened to me,” Vanderbilt said about her past and why Anderson grew up knowing little of it. “So much that was so complicated to even try to define it or explain it. So it just didn’t come up.”

Garbus managed to capture Cooper and Vanderbilt at an almost sublime moment in their relationship, when the information exchange between a mother and son takes on elements of time travel, philosophy and a final sense of where things stand. Both have spent their lives imagining that their late fathers each left behind a letter, addressed only to them, explaining everything and reassuring them of a father’s love. The letters never came, though Vanderbilt insists hers may yet miraculously arrive, after nearly a century of waiting.

Near the end of “Nothing Left Unsaid,” before the two take a car ride out to the cemetery where Wyatt and Carter are buried alongside each other, Cooper speaks of a “faraway look” that he sees in his mother’s eyes — now and also in old photographs. He tears up trying to describe her life of longing and forging ahead. For all that has been said between them, there are still feelings that are impossible to describe.

“Willa Cather said, ‘The heart of another is a dark forest,’ ” Vanderbilt said. “You never really know anybody — really — but after this, Anderson and I have come as close as I think we can.”

Nothing Left Unsaid: Gloria Vanderbilt and Anderson Cooper (108 minutes) airs Saturday at 9 p.m. on HBO, with encores.