Prince Charles with his wife Camilla watch a powhiri at Tuahiwi Marae in Christchurch, New Zealand. Photo / AP

The year was 1975. She was married to another man, Andrew Parker Bowles and he was one of the world's most eligible bachelors. At the time it could have been dismissed as old pals catching up at the polo.

But what few people knew at the time was the heartbreaking and tumultuous story that was playing out behind the scenes.

The couple's early romance has been a captured in the new series of Netflix's The Crown, but not the couple's first meeting.

Of all the places for history to be made, the Stack House flats in the well-to-do suburb of Belgravia, London are unusual. Squat and brown brick, the four unassuming buildings that make up the Cundy Street development are largely forgettable.

But it was here, on one fateful night in the 1971, events were set in motion that would threaten the monarchy and shock the world.

Today, Camilla, Duchess of Cornwall is Prince Charles' devoted wife and partner. She has become one of the royal family's hardest workers, notching up more than 200 official events in 2018 alone and is thought to get on well with the Queen.

All of which is a far cry from the tumultuous, passionate and heartbreaking events of the 1970s, 80s and 90s that transformed Camilla into a household name and hated woman.

There are various rumours about how the couple actually met. One has Camilla introducing herself to Charles, with the show stopping "You know, sir, my great-grandmother was the mistress of your great-great-grandfather – so how about it?"

Another variation of that story has her coming up to him at a polo match and, equally alluringly, purring "That's a fine animal you have there sir."

However, according to a number of credible biographers, including Jonathan Dimbleby who completed an authorised biography of the Prince of Wales, the whole four-decade-long romance started in one place: The Stack House flats.

DEB'S DELIGHT

While the Swinging 60s took over London and indelibly changed the social landscape, for the young members of the English upper classes little had changed. There were debutante balls to attend, foxes to hunt and good marriages to be made. For the daughters of the aristocracy, any sort of substantial secondary education was a rare thing, with most girls being shunted off to Swiss finishing schools to pick up some handy wifely skills and to practice their French.

Prince Charles saw out the end of the youthquake decade as a student at Cambridge University. While other undergraduates experimented with drugs and protested the Vietnam War, the tweed-clad young man was studiously working at his degree and fretting about his future as King.

Keeping an eye on the young royal was the Master of Trinity College, Lord 'Rab' Butler a former Chancellor of the Exchequer and Conservative Party grandee. It just so happened that in 1971, he had a researcher by the name of Lucia Santa Cruz working for him, who was the daughter of the Chilean Ambassador to the UK. Five years older than Charles, she spoke Spanish, French, Italian and English and had two university degrees including one from Oxford.

It was Rab Butler who had the idea of introducing the outgoing Lucia to the Prince and despite their differences, sparks flew. Most royal biographers are in agreement that Lucia was most likely the Prince's first sexual partner and the duo were photographed out and about in London being ferried about in chauffeur-driven cars to black tie dos.

However, at some stage that same year, Lucia traded Cambridge for London and moved into the Stack House building in the Cundy Street Flats development. Living one floor below her was a popular girl about town by the name of Camilla Shand.

"JUST THE GIRL"

One of the most important people in Prince Charles' life was unquestionably Earl Mountbatten, the Duke of Edinburgh's uncle. Throughout his teens and 20s, it was Mountbatten who provided the young Prince with guidance and advice, including telling him that "in a case like yours, the man should sow his wild oats and have as many affairs as he can before settling down".

In 1971, Charles and Camilla could not have been in more different places in their lives. While he had just enjoyed (most likely) his first sexual and romantic dalliance with Lucia, his heart was for the taking. Camilla, however, was already desperately in love. In March 1965 at her 'coming out' party (the event that formerly launched a debutante onto the social scene) she had met a dashing Household Cavalry officer named Andrew Parker Bowles. One year later, at a ball in Scotland in 1966, he asked her to dance and according to biographer Penny Junor, Camilla fell head over heels for the good-looking military man.

Andrew was taken with Camilla but monogamy was not his forte and over the coming years he dated other women including her friends, while also seeing her. By 1971, Camilla was growing tired of Andrew's reluctance to make a commitment and was living in London working as a receptionist for the exclusive design firm of Colefax and Fowler.

She had become firm friends with Lucia, with the women popping in and out of each other's flats and borrowing each other's clothes, according to Junor.

While Lucia and Charles were no longer an item, they remained on good terms. In April 1971, Charles went on an official tour of Japan and when he returned he had a gift for her (a small box). At some stage, Lucia had a light bulb moment, telling Charles she had 'just the girl' for him.

"The Prince was coming for a drink, and I asked [Camilla] to come up," Lucia later told biographer Sally Bedell Smith.

Ever thoughtful, Charles turned up that evening with a second gift with him – for Camilla.

As Junor tells it, Lucia said to the duo "Now you two be very careful, you've got genetic antecedents … Careful, CAREFUL!" (What she was referring to was the fact that Camilla's great-grandmother Alice Keppel had been Charles's great-great-grandfather King Edward VII's mistress for many years.)

The attraction between the pair was reportedly immediate, impressed by her easy going way and apparent indifference to his royal status.

THE ANNE FACTOR

Complicating Charles and Camilla's romance were two people – Andrew Parker Bowles and Charles' sister, Princess Anne.

Andrew and Anne had been enjoying a fling of their own, with the Princess even inviting him to stay at Windsor Castle. While the romance could never have blossomed into marriage (Andrew was a Roman Catholic and under the then current act of succession no heir to the throne could marry a Catholic) the pair are reported to have had real feelings for one another.

One question that biographers have grappled with is just how much was Camilla's affair with Charles an attempt to make Andrew jealous?

SOCIETY WEDDING

Camilla's patience and tenacity paid off. After an intense romance with Charles, he was forced to say goodbye to her in late 1972 when he embarked on an eight-month naval tour of the Caribbean. While he was intensely in love with Camilla, she had her heart set on marrying Andrew.

In March 1973 when Charles was in the West Indies, he received a letter that broke his heart – Andrew had finally proposed and they were to marry. Charles wrote to Camilla, begging her not to marry him, but to no avail. On July 4, 1973, in front of a huge congregation that included the Queen Mother, Princess Margaret and Princess Anne, Camilla became Mrs Andrew Parker Bowles.

In a letter at the time Charles wrote, "I suppose the feeling of emptiness will pass eventually."

It would be years before Charles and Camilla were reunited.

After all, the course of true love never did run smooth.