An American arrives in the Royal Family and causes uproar, bitterness and recriminations that will echo down the years.
Yes, Meghan Markle, but almost a century earlier Wallis Simpson, without whom our queen would not be sitting on the throne.
The 1936 abdication crisis set in motion events that have an eerie resonance with this new royal storm.
Not only did it mean the Queen’s father became king, it taught a young Elizabeth that protecting the monarchy reigns supreme.
It is arguable her father gave his life for the Crown – his wife, later the Queen Mother, believed this to be the case.
She never forgave Wallis Simpson for condemning him to kingship, the stress shortening his life and sentencing her to almost half a century of widowhood.
With her beloved father setting such an unselfish example, it is no surprise in the decades following his death the Queen has placed safeguarding the monarchy above all other things.
Clive Irving, author of The Last Queen: How Queen Elizabeth II Saved The Monarchy, says: “Above everything is her desire to preserve the monarchy.
"And the monarch isn’t the monarchy, the monarchy is the institution.
“She understands that the monarch serves the institution.
“It’s astonishing that the Queen has been able to hold together the institution for as long as she has.
“She’s gone through some important and visible changes, and each of the changes has been driven by the way she’s handled particular crises.”
Any time anyone has tried to put their own interests ahead of those of the monarchy, the Queen – or those around her – pushes back to preserve its reputation.
Her life has been one of service and so she expects the rest of The Firm to put the Crown first too.
Elizabeth became Queen with the unexpected death of her father, aged 56, just 11 years after he took the throne.
Her crisis management techniques were then honed by one of the best, says Clive.
“She was thrust into the part in 1952, very inexperienced in the world and not at all politically skilled.
"But she was tutored by Winston Churchill, who luckily was her first Prime Minister.”
The royal author believes the “first real tester” of the Queen’s resolve was her sister Princess Margaret’s romance with Battle of Britain pilot Peter Townsend, another story with echoes of the Harry and Meghan saga.
As a divorced man he was deemed inappropriate and the relationship could have damaged the reputation of the Crown.
So, no matter her own sister’s feelings in the matter, he had to go.
Whether it was the Queen herself or the “men in grey suits” at the palace – as Princess Diana famously called them, any chance of scandal was erased.
It was when the popularity of the Royal Family was riding high, after millions watched the fairy-tale wedding of Charles and Diana, the birth of princes William and Harry and the marriage of Prince Andrew and Sarah Ferguson, that the Queen hit upon the toughest time of her reign.
In the Queen’s “annus horribilis” in 1992, Prince Andrew separated, Princess Anne divorced, Andrew Morton’s book on Diana was published, a fire gutted Windsor Castle and the Prince and Princess of Wales announced their split.
The Queen had expected Diana to fit into the Royal Family because she was born into privilege.
Royal expert Ingrid Seward wrote: “The Queen believed her future daughter-in-law knew what was expected of her.”
When Diana gave the BBC interview to Martin Bashir in 1995, when she revealed there were “three of us” in her marriage, the Queen knew she had to act.
Watched by 22.8million viewers, Diana described the prince’s camp as the “enemy” and said that the monarchy was in desperate need of modernisation.
A letter handwritten by the Queen arrived weeks later.
Delivered in person by a royal messenger, it left Diana in no doubt about the Queen’s annoyance.
“I have consulted with the Archbishop of Canterbury and with the Prime Minister and, of course, with Charles, and we have decided that the best course for you is divorce,” the Queen wrote.
That was it – Diana was out.
It was the death of Diana in August 1997 that rocked the Royal Family more than any event since the abdication – and for once the Queen got it wrong.
So, Clive believes, it is important she gets it right with Diana’s son and Meghan.
The “men in grey suits” seem to be doing their worst with allegations of bullying flying from the palace.
He says: “She fully and clearly understands that whatever the mess underneath her, the monarch had to be above it.
"You see this right now, with this enormous mess going on, that you won’t find the Queen’s fingerprints on anything.
"She must be very preoccupied with Philip’s condition, he’s very important in her life and one wonders if she’ll want to go on being Queen after he dies.
“It’s going to seem like another episode in this great royal soap opera that she’s going to want to stay out of.
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"She is above all that, she’s safely in her bubble. However this unfolds, no one is going to blame the Queen.
“I suspect that is how she wants it.
"If the monarch is untouched, the monarchy will survive.”