Unbelievably, Princess Diana’s wedding dress has never been the subject of a documentary, film or TV show despite the bride’s fame, love of fashion and enduring mystique.

Britain’s Channel 5, a division of Paramount Global, is about to change that with a TV documentary that takes a close look at the origins of the twinkly, frothy design in “Secrets of Diana’s Wedding Dress,” which premieres on Saturday at 9 p.m. local time.

The one-hour show covers the rapid rise of the Emanuels from young, married Royal College of Art grads, to celebrity — and royal — dressers. Bianca Jagger was the Emanuel’s first big customer, buying designs straight from their graduate show and wearing one of their white dresses with silk roses to Studio 54.

She sparked a craze. Soon Carolina Herrera, Princess Anne and stylish duchesses up and down the country were calling the couple’s Brook Street studio in Mayfair to place their orders.

The equally young Lady Diana learned about the Emanuels through the editors at British Vogue, who were helping shape her image at the time, and soon she was calling them on the phone, like any other customer, and asking, “Would you do the honor of making my wedding dress?”

After she put down the phone, Elizabeth screamed and for a moment forgot about one of her other posh clients who was waiting downstairs, half-dressed, for her own wedding dress fitting. Over the next months, there would be many screams, and high-wire moments, including on the wedding day itself when Diana emerged from her horse-drawn glass coach with more than a few unwanted creases in her dress.

“My heart stopped,” Elizabeth says, recalling the moment she saw Diana begin to ascend the steps of St. Paul’s Cathedral in London. David remembers that the dress was looking “rather distressed” until the bridesmaids got to work smoothing it out and and arranging the 25-foot-long, lace-edged train.

India Hicks, one of the bridesmaids, says “manhandling the train was virtually impossible.” Its length was meant to match the grand scale of St. Paul’s, and it was so unwieldy that the Emanuels were forced to roll it up — and stuff it beside Diana after she settled herself in the coach that would take her to church.

The documentary is brimming with detail and also spotlights the Greek-born Nina Missetzis, who hand-sewed the dress and worked closely with the Emanuels’ small team of seamstresses, one of whom was Elizabeth’s mother, Betty, a needlepoint whizz.

In separate interviews, the Emanuels — who would later split personally and professionally and pursue separate careers — also discuss the Olympian efforts they made to keep the dress secure and away from prying eyes.

They referred to their top-secret client as “Deborah Cornwall,” and — as the dress was nearing completion — brought a wardrobe-sized safe into the studio so they could store it safely.

They tossed decoy fabrics into the garbage bins to deter nosy tabloid reporters who regularly searched them, and created an emergency backup dress, without ruffles, in case the real one was damaged or stolen. The backup mysteriously disappeared from a sample sale rail not long after the wedding.

It’s also brimming with fun reveals. Barbara Daly, Diana’s makeup artist on the day, says the soon-to-be-princess spilled perfume down the front of the dress before leaving for church and was terrified that the Emanuels would notice. They didn’t — and neither did anyone else.

David recalls — with slight horror — the posse of “nipping corgis” that greeted him at Clarence House, where the final fittings took place. He also remembers emerging, on his hands and knees, from under Diana’s ample underskirts — he was making final adjustments — only to see Queen Elizabeth the Queen Mother looking at him quizzically.

Elizabeth even remembers Queen Elizabeth getting hangry during the post-wedding photo shoot, and urging her snapper cousin, Lord Patrick Lichfield, to wrap things up so they could head to the wedding breakfast.

Although lighthearted, the documentary doesn’t shy away from some difficult topics, such as Diana’s anxiety about Charles’ old girlfriend (and now wife, Queen Camilla) and her ever-shrinking waistline. It measured 23 inches by the time she walked down the aisle, and Elizabeth says they had to remake the bodice every time she went to a fitting.

Making that dress changed the Emanuels’ lives. They were grateful for the break, and for the clients who came flooding in following the wedding — including Jerry Hall, Daryl Hannah, Anjelica Houston, Sigourney Weaver and Faye Dunaway, in addition to numerous British celebrities.

They were also under constant pressure to achieve, and that took a toll. As with other fashion designers here, they struggled with financing and finally went their own ways.

“The dress has dominated my life,” says Elizabeth, who kept the diaries of those years, all the sketches she made and even the leftover veiling and ivory taffeta she used for the dress. She shows it all in the documentary.

In an interview, she says it was high time the story of the dress was told.

“I had wanted to do a documentary like this for many years. There are loads and loads of documentaries about Diana, quite rightly, but this is a story within a story that’s never been told, and it’s a story of teamwork,” she says.

Elizabeth believes the circumstances were unique to the time, and still can’t believe the tiny atelier, run by two young designers, actually pulled it off — without any major disasters.

“We were just young students, who suddenly [were] becoming two of the most famous designers in the world, virtually overnight. When you think of royal wedding dresses today, they’re made by big companies, with lots of staff and everything set up. This was just David and myself and our tiny team, and I wanted to give them credit, because their names have really not been talked about,” she says.

Elizabeth says she loved watching the documentary. “The footage they managed to get together is fantastic. Some of it I didn’t even know existed, so it was really fun for me to watch, and to see Nina Missetzis, who made the dress. She was the first seamstress who we took on when we set up Emanuel.

“We knew it had to be her making the dress, and it became her baby, really. When it came time to release it, we had to virtually drag it from her. She didn’t want to let it go,” says Elizabeth, who doesn’t want to release it either.

She’s already talking to publishers about doing a book about the dress, and says she’d love to do an exhibition, too, and maybe even a hologram.

“Wouldn’t it be wonderful to have a hologram made and to show the dress forever sparkling, and to see the veil in motion a bit, and recreate that effect from that day?” Elizabeth asks. “Plus, a hologram would be portable. They wouldn’t have to keep getting the dress out the cupboard,” to put it on display.