The Liberal Democrat win in Chesham and Amersham is undoubtedly a famous by-election victory, seemingly from nowhere, with repercussions which will ricochet for a long time to come.
It's their best by-election result for almost 30 years. But what is its significance?
Conventional wisdom wrong again
It's a bad day for groupthink - MPs of all stripes, pundits and the media. The result is all the more painful for the Tories because they didn't see it coming.
Even this week, Tory MPs thought they would still win by a slim majority, probably in four figures.
Nor were Lib Dems confident: I spoke to senior figures earlier in the week who were divided over whether they would get the result they wanted.
The media and the pundits all largely missed it, too, with one leading journalist saying he would eat his hat if the Tories lost. Jim Pickard, chief political correspondent at the Financial Times, has since tweeted a video of himself doing just that.
The last few years have seen Labour routed in Scotland and the Tories winning in the Tees valley - including Sedgefield, Tony Blair's old constituency.
Sinn Fein could win the most seats in Northern Ireland if there is a snap Stormont election. For all the headlines following May's local elections, predicting another decade of Tory hegemony, the message today is that you can't take the British electorate for granted.
The Lib Dems aren't dead
Former Liberal Democrat leader Jo Swinson walked away from a party on life support after the December 2019 general election, despite entering the contest thinking she might end up as prime minister.
Now her successor, Ed Davey, is celebrating a 25-point swing from the Conservatives to the Lib Dems, their third best by-election result and the kind of stunning victory not seen since 1993.
The Lib Dem campaign was agile, capitalising on concern about the Tory planning reforms and HS2, and they succeeded by arguing that it was a two horse race, meaning there were almost no votes for any other party.
No one, not even Davey, expected a majority of more than 8,000.
Worst by-election result since the war for Labour
Labour will argue it's not all bad news for Keir Starmer.
With just 622 votes - 1.6% of the electorate - there was no sign whatsoever of any affinity for the Labour party, or any advance, in Tory heartlands.
That message is stark. The consolation is that the first-past-the-post electoral system means Labour doesn't need to do well in places like Chesham to have any chance of winning a general election.
Indeed, a Liberal Democrat revival, taking seats across from the Tories across the South and West of the country, increases Labour's chances of getting into Number 10.
In 2015 the reverse happened: swathes of Liberal Democrat seats under Nick Clegg turning blue - ensured David Cameron got a majority.
Labour will cling onto this small comfort, although the party still needs to show they can win Tory votes. If they lose Batley and Spen, in two weeks' time, this glimmer of optimism will be extinguished.
The incumbent lost again
Six weeks ago in Hartlepool, a seat which had always been held by a Labour MP, the Tories managed to pull off their own famous by-election victory by capitalising on resentment of long time Labour MPs failing to deliver, and presenting themselves as the "change" party.
It was quite a coup, given that there has been a Tory in Downing Street for 11 years, and they will try and repeat the trick in Batley on 1 July.
In Chesham and Amersham, the Liberal Democrats attacked the "tired" Tory government and presented themselves as a vote for change.
This should worry the Tories since there are far more seats in traditional heartlands - sometimes called the "blue wall" - than there are "red wall" northern seats where they won for the first time in 2019.
And so much of Boris Johnson's energy and spending has been in the north, meaning many southern Tories already felt unloved.
This starts all sorts of awkward conversations
It is just one seat, but already Tories are starting conversations about what message should be heard from the result.
Cabinet ministers wonder if this will be a wake-up call. Some wonder whether Boris Johnson has become too complacent.
Liberal Democrat leader Ed Davey claims he can attack dozens of seats across the "blue wall" Tory heartlands of the south and west, some of which used to be Lib Dem strongholds.
However, by-elections allow huge resources to be funnelled into one seat, so it does not automatically follow that this pattern would be repeated at a general election.
One interesting point of agreement between Tories and Lib Dems is that neither put this result directly down to Brexit. Whether true or not, both sides want to move on from the subject which has bedevilled British politics for six years.
The lessons are different to the lessons in the May local elections
In the May elections the Tories surged in the north, but there were signs that its electoral coalition was fraying in the south.
However it was to a far lesser degree, with Tory opposition in England split between Labour, Lib Dems and Greens, diluting its impact.
Chesham shows tactical voting is alive and well, and there is an appetite among certain voters to form an anti-Tory coalition.