King Charles III seemed upbeat as he strolled to church at Sandringham on Sunday with Queen Camilla by his side, giving well-wishers a cheery wave.
Yet little over 24 hours later, days after successful surgery for a benign prostate condition, the bombshell dropped: His Majesty has cancer.
Buckingham Palace said the 75-year-old King, who left Sandringham for London yesterday, had been advised to postpone public-facing duties as he begins a course of treatment.
Less than 18 months after the death of Queen Elizabeth II, the monarchy faces a seismic shock as the King steps back from the spotlight and other senior Royals step up to hold the fort.
That will mean the Prince of Wales - whose wife Kate is currently recuperating from abdominal surgery - the Princess Royal and the Duke of Edinburgh will take his place.
The sovereign's diary included the Commonwealth Day service at Westminster Abbey on March 11, a state visit to Canada with Queen Camilla pencilled in for May, the 80th anniversary of the D-Day landings in June and the Sandringham Flower Show in July.
The King's absence from the latter will send shockwaves across Norfolk, for the much-loved event of which he is the Royal patron sees him and the Queen touch base with many of the charities and organisations which he supports across the county.
They range from beekeepers to the King's Lynn Osteoporosis Society, along with the Sandringham Association of Royal Warrant Holders - businesses large and small, which supply the Royal Estate including bakers, builders, caterers, carpenters, electricians and horticulturalists.
In six months' time - depending on the King's prognosis - it may well be the Prince and Princess of Wales who arrive at 11am on the dot in the summer sunshine in the horse-drawn carriage.
Buckingham Palace has not so far disclosed the nature of the King's cancer or the treatment which he is undergoing.
Cancer has no respect for social standing. King George VI, the monarch's grandfather, passed away from the ravages of the disease at Sandringham 72 years ago today.
But Charles III can draw hope from the fact the disease was discovered early, while his healthy lifestyle should bolster his defences as his doctors go to work.
After they finish their work and it has the hoped-for successful outcome, His Majesty's thoughts will doubtless turn to where he will go to recuperate before he fully resumes his Royal role.
Initially, he is flying back to Sandringham with Queen Camilla, the Norfolk estate he has been stamping his mark on since his late father, Prince Philip, retired from public life in 2017.
He has overseen the roll-out of organic farming methods, the planting of hedgerows and measures to encourage wildlife such as the threatened curlew. Rare breed cattle are due to be introduced later this year.
The King has been hands-on in the running of the 20,000-acre estate so beloved by his forbears, paying frequent visits to Norfolk.
He will be continuing with behind-the-scenes work on his red boxes of state papers while he stays at Sandringham.
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