"I declare, before you all, that my whole life, whether long or short, shall be devoted to your service."
That Queen Elizabeth II has kept her vow - given in a speech in South Africa as a young woman of 21 - is indisputably evident today as the 86-year-old monarch remains in perpetual motion in her devotion to her British and Commonwealth subjects.
Millions know her, love her and draw comfort from her decades of Gibraltar-like durability and her positive influence in the ever-shifting global drama. "She is our most familiar enigma," says the ever-effusive Andrew Marr, the presenter of The Diamond Queen, a three-part documentary and the BBC's flagship tribute in the 60th year of the queen's reign.
While her loyal subjects and fans are fussing up a storm, as might be expected, the queen cares not a whit about fame, according to her grandson and future heir Prince William, the Duke of Cambridge.
"I think she doesn't care for celebrity...and she really minds about having privacy in general. And I think it's very important to be able to retreat inside and be able to collect one's thoughts and collect your ideas...and then to move forwards," he says in the programme.
"[It is] a very tricky line to draw between private and public and duty, and I think she's carved her own way completely. She's not had a blueprint."
William is but one of many regal grandchildren who gets a rare on-air say about what it must have been like to become queen at the age of 25. The series also boasts interviews with Prince Harry, Princesses Beatrice and Eugenie, Zara Tindall and Peter Phillips, the Earl of Wessex, the Duke of York, the Princess Royal, David Cameron, Tony Blair, Sir John Major and Barack Obama.
In the first one-hour instalment, with remarkable archive footage, Marr tells the childhood story of the young girl who never expected to reign.
He looks closely at the influence of her modernising grandfather King George V and of her father King George VI, who found himself thrust on to the throne by the abdication of the love-smitten King Edward VIII.
Marr also examines the queen's role as the head of state. He follows her to the US and the Middle East to assess the worldwide impact of the royal family on British trade and international relations.
The Middle East segment includes the queen's November 2010 visit to Abu Dhabi, where the modest, respectful monarch donned a beekeeper-style shawl and hat and went shoeless to meet Islamic students at the Sheikh Zayed Mosque.
In the second programme, Marr looks at how she has modernised the monarchy, from the abolition of the presentation of debutantes in 1958 to the ultramodern wedding of William and Catherine
The "annus horribilis" of 1992 is covered, as is the queen's visible sadness when her beloved Royal Yacht Britannia was scrapped by Tony Blair after he came to power in 1997.
In the final hour, Marr highlights the defining moments of the queen's reign - her 1952 accession to the throne and coronation 16 months later, her at-times-testy relations with the media and, what many view as her enduring triumph, the Commonwealth.
Some of the most touching aspects of this documentary series are the glimpses it affords into her family life, dispelling some of the enigma, such as when Harry, 27, recognises the contributions of his 90-year-old grandfather Prince Philip, the Duke of Edinburgh, to the monarchy and his wife of 64 years.
"Regardless of whether my grandfather seems to be doing his own thing, sort of wandering off like a fish down the river," says Harry, "the fact that he's there - personally, I don't think that she could do it without him, especially when they're both at this age."
The Diamond Queen will be broadcastwat 9pm on Wednesday (Part 1), 9pm on Thursday (Part 2) and 9pm on Friday(Part 3) on BBC Entertainment