Princess Diana recounts her engagement to Prince Charles in exclusive clip

What was running through Princess Diana’s mind when Prince Charles proposed to her? Was she excited? In love? Filled with trepidation at joining the royal family?

Diana: In Her Own Words, a new two-hour documentary from National Geographic, aims to answer those questions using audio recordings from a series of secret 1991 interviews in which Diana discussed her marriage proposal (and much more) with journalist Andrew Morton for a book about her life. In them, she revealed the more heartbreaking aspects of Charles’ proposal — and that she gave the offer a lot of thought before accepting.

“I remember thinking, ‘This is a joke,'” Diana said in the recordings. At the time, she already had realized the nature of Charles’ relationship with Camilla Parker Bowles — with whom he shared a decades-long affair — and said knew deep down that she would never be queen. But she said that she accepted the proposal because of her belief that Charles was in love with her — and her desire to help people.

“In my immaturity, which was enormous, I thought that he was very much in love with me, which he was. He sort of had the besotted look about him, looking back at it, but it wasn’t the genuine sort. You know, ‘Who is this girl who’s so different?’ But he couldn’t understand it,” she said. “It was like a call to duty, really. Go and work with people.”

Diana accepted that call to duty with aplomb and, following her fairy-tale wedding, became a beloved icon known as the “People’s Princess,” leaving an extensive legacy of charity work.

Diana: In Her Own Words, which iscompletely narrated in the late royal’s voice, “weaves Diana’s thoughts and feelings seamlessly with archival footage and photography. It’s a personal account of her difficult childhood, struggles with bulimia, humanitarian passions and of course, her courtship and marriage to HRH Prince Charles and the births of her beloved children,” according to NatGeo.

The documentary airs Aug. 14th at 9 p.m. ET. Watch the clip above for more.