Sir Michael Fallon used to be regarded as one of the safest pairs of hands in the Cabinet, but he appears to have dropped the ball in his private life.
He admitted as much in his resignation letter to Theresa May when he said he had "fallen below the high standards we required of the armed forces".
He was held in such high regard that a little over 24 hours before he resigned, Ken Clarke gave him glowing praise after the Defence Secretary admitted touching radio presenter Julia Hartley-Brewer's knee 15 years ago.
"Michael Fallon is an excellent influence in the Cabinet, he is a very good Defence Secretary, a very good presenter of the Government's case in these fervent times," said Mr Clarke in a radio interview.
As Theresa May said in her letter accepting his resignation, Sir Michael served four Prime Ministers.
He was nearly always capable and reliable in radio and TV interviews when the Government was in trouble and an attack dog with real teeth against Labour.
That's why inside the Palace of Westminster, MPs reacted with shock to news of Sir Michael's bombshell resignation, which came minutes after the end of a bad-tempered Commons debate on disclosure of Brexit documents.
Seconds later I bumped into Jeremy Corbyn, who was dashing along a corridor with an aide.
"Have you heard about Michael Fallon?" I asked him. "Amazing!" he replied, smiling. He obviously had.
I asked the Labour leader if I could tempt him into doing an interview for Sky News.
"Not yet," he said as he disappeared up a staircase, clearly in a hurry to get somewhere.
There was no such hesitation from the Liberal Democrats' wily chief whip Alistair Carmichael, who served in the coalition cabinet with Mr Fallon – as he was then – from 2010 to 2015.
"Theresa May's government is falling apart at the seams," he said. "As we face the biggest negotiations in the modern history of this country her Cabinet can't even agree to sit around the same table.
"Theresa May lacks the authority to lead this country and her government is in meltdown."
Losing one Cabinet minister isn't quite meltdown. Yet. But if more were to quit in this Westminster sex harassment scandal, it could soon be.
What if Damian Green, facing a Cabinet Office investigation, becomes another Cabinet casualty?
When he talks about the biggest negotiations in modern history, Mr Carmichael means Brexit.
And the Cabinet is already deeply split on its strategy, between Remainers, single marketeers, customs unionists, cliff-edgers and no-dealers.
So now the Prime Minister is plunged into a Cabinet reshuffle that she has been postponing since she lost her Commons majority and her authority in the June election.
After the 8 June poll, she was so wounded that the only change was Michael Gove's comeback.
Does the PM now just carry out a limited reshuffle and replace Sir Michael with a current member of the Cabinet or a rising star among the ranks of ministers of state? Probably the former.
Early front runners are May favourites Ben Wallace, the security minister, Penny Mordaunt, a former defence minister and Portsmouth MP, old university friend Sir Alan Duncan and current defence minister Tobias Ellwood.
Or does she opt for a bold, radical reshuffle clearing out the so-called dead wood and promoting young talent and rising stars?
Tom Tugendhat, newly elected chairman of the Foreign Affairs Select Committee, is the most upwardly mobile of the rising stars and tipped as a future party leader.
He was spotted talking to Transport Secretary Chris Grayling in Westminster after the Fallon resignation.
Mr Tugendhat, a former Army officer who served in Iraq and Afghanistan, would be the bold choice as Defence Secretary.
Mr Grayling, an old hand and May ally who goes back with the PM to their days on Merton Council in south London, would be a safety-first, unimaginative choice.
With Tory Eurosceptics demanding the sacking of Chancellor Philip Hammond over his Brexit stance, Sir Michael was a possible replacement at the Treasury.
He made his name as a hard-hitting interrogator on the Treasury Select Committee, after all.
Now 65, Sir Michael is probably too old to make a Cabinet comeback.
Theresa May could decide to move out some of the older members of her Cabinet, or even take the bold and potentially dangerous step of demoting Boris Johnson from the Foreign Office.
Critics – including at least one member of the current Cabinet – used to say of the PM: "Theresa May, or maybe not. She used to be indecisive, but now she's not so sure."
A clue as to how decisive or indecisive will be in how swiftly she replaces Sir Michael.
When Michael Heseltine stormed out of the Cabinet so dramatically over the Westland helicopter scandal in 1986, Margaret Thatcher had already decided to replace him with George Younger and did so immediately.
Will Theresa May dither or act swiftly? Insiders predict a one out, one in reshuffle.
But that would confirm the PM's weakness. And, many MPs would argue, it would also be a missed opportunity to refresh her stumbling Government.