For decades, the bulky plan for Operation London Bridge has sat in drawers around London, gathering dust and shortbread crumbs. (Eating biscuits and reading Evelyn Waugh, that's what Brits of a certain sort – male, pinstriped, public school-education – do all afternoon right?)
Everything had been taken into account, from the fact that pallbearers would need to practise carrying her lead-lined coffin to someone having the job of covering Big Ben's hammer with a leather pad precisely 1.1cm thick for Her Majesty's funeral.
But as has become painfully apparent there is one very serious detail no one had bothered to consider or to put any contingency plans in place for: that Harry and Meghan, the Duke and Duchess of Sussex could be on British terra firma when the sovereign drew her last breath.
Over the past four days, since the Queen's passing at the age of 96, Buckingham Palace has proven what a meticulously-run, gold-braided machine it can be, with hundreds, if not thousands, of military personnel, Privy Counsellors, dignitaries, and trumpeters all having been corralled and put in place, seemingly effortlessly.
Here's where the big, fat "except" comes in …
Except for the Harry situation. Clearly no one had taken into consideration how things would play out if the sixth in line to the throne was, quite literally, just down the road from his family as this was all unfolding.
Now, new details have emerged about how the younger prince was kept out of the loop and why he was not on the same private jet as his brother Prince William, that made a mad dash north last Thursday afternoon, UK time, from Windsor to Scotland.
By Thursday morning, UK time, the Sussexes had been in the UK for a number of days, having flown in for several days of charity events, during which they had precisely zero intention, reportedly, to see his family. Goodo then.
The first two of these outings went swimmingly well with the requisite applause, all probably making for super Netflix content and then on the Thursday night, the duo had been set to attend the WellChild Awards.
Things went off the rails at 12.34pm when the Palace put out a highly alarming statement about the Queen, saying "doctors are concerned for Her Majesty's health" and that she was "under medical supervision".
Within hours, photos of a grim William, Prince Andrew, Prince Edward and his wife Sophie, Countess of Wessex zooming through the gates of Balmoral to be by the Queen's side were running on news websites the world over.
By the time Harry was photographed in the exact same place, at just before 8pm and in the pitch black, the Queen's death had already been announced.
Those images of the Duke of Sussex, travelling alone, his face bleak, could not have been more of a contrast to those of the shots of William's Range Rover stuffed with HRHs. An isolated, forlorn figure versus a sombre but united front.
Since then, the melodrama surrounding the Sussexes' reintroduction to royal life, albeit temporary, has proven to be an ongoing distraction from the death of the Queen and the accession of King Charles III.
So how the dickens did this mess come to pass?
Let's rewind to last Thursday lunchtime, with the Palace's Queen statement having hit.
It only took 13 minutes for Kensington Palace to announce that William would be travelling to Scotland, with the announcement coming out at 12.48pm. It was over an hour later, at 1.55pm that the Sussexes' spokesperson put out a similar statement saying they too were heading to Scotland.
Today, Camilla Tominey, the journalist who got the scoop of the decade back in 2016 when she revealed Harry was dating the Suits star Meghan, has provided new insight into the events of last Thursday in the Telegraph.
As William stepped on board a private jet at RAF Northolt, about 2.30pm, "Harry hadn't even been called," Tominey writes. (Keep in mind here, both men are currently living roughly 120m apart from one another in their respective homes on the Windsor estate.)
The Telegraph reports that this Harry omission "was because the heir to the throne had initially been summoned by his father in his constitutional capacity as a Counsellor of State." Which would make perfect sense, if not for the fact that Harry is a Counsellor too. (As is Prince Andrew who also made the trip with William.)
Per the Telegraph: "It seems that while the family knew the Queen was gravely ill when they took off from RAF Northolt on Thursday lunchtime, they didn't expect her to deteriorate so rapidly – with Charles even thinking that he and William might have been able to host the virtual Privy Council meeting she had postponed the previous day."
At 3.52pm, William's plane landed at Aberdeen airport and he and his uncles and aunt arrived at Balmoral, the favourite of his grandmother's homes, at 5.11pm.
It was only at 5.35pm that back outside of London, Harry's chartered Cessna took off from Luton airport. (Interestingly, he flew from a commercial airport that is a 45-minute drive away from Windsor unlike William et al who got to use an air force base which is closer to their homes.)
At 6.31pm, the Palace confirmed the Queen's death with Harry still in the air. He arrived at the 20,000ha estate at 7.52pm.
Not helping this business is the fact that it has now emerged, via the Daily Mail, that Harry did not even see his father or brother, the impeccably connected Richard Kay has reported. Kay writes that Charles and William dined together at Birkhall, the King's home, while Harry ate with his uncles and aunt.
"Two dinners were being hosted on the royal estate that night and there was a clear divide: One was for the new king and his heir, the other was for the rest of the family," an insider told Kay.
Let's recap then: Harry was not called or told about his grandmother's declining health, the Palace went ahead with announcing Her Majesty's death even though they knew that Harry was en route to Balmoral and then Charles and William did not have dinner with Harry, though whose choice that was we don't know.
None of this is a good look for a royal family who is, now more than ever given the world is closely watching, obsessively, acutely focused on public perception and its image.
Sure, Harry and Meghan's kvetching about their many injustices they say they faced during their royal careers have seen the support for them plummet to subterranean depths in recent years in the UK but that does change the fact that the Palace's handling of them in the last few days has proven truly lacking.
If William was alerted to his grandmother's declining health in his role as a Counsellor of State, then why wasn't Harry given his status as a Counsellor too?
Then, once it became apparent that the duke was going to race to Scotland to see the Queen, why wasn't the choreography of the situation better managed? The optics of the way the two lots of arrivals played out has only strengthened the Sussexes' general characterisation of the Palace as an unfeeling, compassion-less institution.
Basically, the Palace has played entirely into Harry and Meghan's hands with their chaotic handling here and they have run the risk of coming across as petty and unnecessarily unkind. Whether intentional or not, their sidelining of Harry reflects poorly on them.
Harry might not have acquitted himself in recent years with much dignity or respect for The Firm but he unequivocally loved his grandmother or time and again told the world how much she loved him right back.
In April he said during a TV interview (where else?), "We have a really special relationship. We talk about things that she can't talk about with anybody else."
Sadly, like so many of Harry's ties to his family, this "special relationship" is now over too.