At the time of Diana's death, the Duke of Cambridge was 15, and Harry was only 12. The two are hoping to now "stand up for her name" and do their duties "as sons" in protecting her and reminding everyone "of the character and person that she was."
According to BBC News, the 90-minute documentary will feature William and Harry speaking about the week following their mother's death, "from the moment they heard the news to the day of her funeral."
"Part of the reason why Harry and I want to do this is because we feel we owe it to her," William explains in the film. "I think an element of it is feeling like we let her down when we were younger. We couldn't protect her."
"I think it's never going to be easy for the two of us to talk about our mother, but 20 years on seems like a good time to remind people of the difference that she made, not just to the royal family, but also to the world," adds Harry.
He continues on, explaining that he and his brother are still so thankful for the outpouring of love and support they received following the tragedy.
"It was beautiful at the same time, and it was amazing," Harry says. "Now, looking back at it, it was amazing that our mother had such a huge effect on so many people."
"When you're that young and something like that happens to you, I think it's lodged in here, there, wherever — in your heart, in your head," he continues. "And it stays there for a very, very long time."
BBC News also reports that the documentary will include interviews with Diana's closest friends, political figures and journalists, many who are speaking out about her death for the first time.