They were two boys impatient to get back to playing, with little time for a phone chat with their vacationing mom.

Two decades later, they still have regrets about the rushed nature of what no one expected to be their final call with their famous mother: Princess Diana.

The boys — Prince William and Prince Harry, now adults — reflected on their regret about that call in “Diana, Our Mother: Her Life and Legacy,” the most recent biopic about Diana. It airs Monday, a month before the anniversary of her death.

It’s a rare glimpse into the sons’ mostly private mourning about that missed opportunity.

“Harry and I were in a desperate rush to say goodbye, you know, ‘See you later.’ … If I’d known now, obviously, what was going to happen, I wouldn’t have been so blasé about it and everything else,” William, the 35-year-old future king of England, said in the documentary. “But that phone call sticks in my mind, quite heavily.”

Harry echoed his brother’s thoughts.

“Looking back on it now, it’s incredibly hard. I’ll have to sort of deal with that for the rest of my life,” Harry said. “Not knowing that was the last time I was going to speak to my mum. How differently that conversation would have panned out if I’d had even the slightest inkling her life was going to be taken that night.”

The show, one of many that will air ahead of the Aug. 31 anniversary, airs at 10 p.m. Monday on HBO.

According to an HBO synopsis: “The film offers a fresh and revealing insight into Princess Diana through the personal and intimate reflections of her two sons and her friends and family, many of whom have never spoken publicly before, to bring together a unique portrait of an iconic person who touched the lives of millions.”

Diana, Princess of Wales, was killed in a crash in the Pont de l’Alma tunnel. Henri Paul, the driver of the Mercedes that she and her companion were in, lost control of the car. He was reported to have been drinking.

A month after Diana’s death, Prince Charles praised his sons for showing “enormous courage and the greatest possible dignity,” according to The Washington Post’s Dan Balz. Charles said the boys would “always feel that loss.”

They were spending the summer with Charles and their grandmother Queen Elizabeth II in Scotland when they were summoned inside for a phone call from their mother.

Princess Diana, Prince William and Prince Harry visit the Thorpe Park Amusement Park. (Julian Parker/UK Press via Getty Images)

But the princes’ mourning has occurred mostly out of view of the public, which had been consumed with Diana’s actions before her death and sprouted numerous conspiracy theories afterward.

The biopic focuses on that intense media scrutiny and the issues Diana championed, according to The Washington Post’s Hank Stuever. She campaigned against land mines and visited AIDS patients when no one else would.

The documentary also deals with Diana’s up-and-down relationship with the media.

William said the only times he remembers his mother crying had something to do with the media.

Those memories have shaped how much William has let the world see of his inner domain, Stuever wrote.

“Harry and I lived through that, and one lesson I’ve learned is you never let in too far, because it’s very difficult to get them back out again. You’ve got to maintain a barrier and a boundary, because if both sides cross it, a lot of pain can come from it.”

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