Just as the Scottish Premiership title race looked like it was alive again, it was buried on the spot by a smash and grab Rangers win at Ibrox.
A 1-0 win in Saturday's Old Firm put Steven Gerrard's team into a lead that is beyond commanding. 19 points, by a team who have so far dropped just four in 22 games. It's unassailable, even if Celtic win each of their three games in hand.
As the dream of a tenth successive title collapses around them, it feels like the end of the world for the defending champions. After nine years of relentless effort, the reset button has just been pressed, sending them crashing back into a reality where one title is enough of a challenge; let alone ten.
They've been living in the clouds for the last nine years, but now it's time for Celtic to return to the real world. A world where they actually need a strategy, beyond just being Celtic, if they want to win trophies on a regular basis.
Ten in a Row is something they have been desperate for ever since Rangers won their ninth on the bounce in 1997. Since their rivals were banished from the top flight in 2011, giving them a head start and a clear run at the fabled accomplishment, the idea has consumed them.
And while that end goal has been a driving force at times, the nature of its existence has bred complacency. A habit of winning is rarely a bad thing but Celtic's own dominance tricked them into believing they were genuinely infallible, no matter what they did.
So when Brendan Rodgers walked away in 2019, they felt as if appointing Neil Lennon was a perfectly fine thing to do. What's the worst that could happen?
Recruiting the Irishman in the first place was a predictably poor decision, and a sin that will never be forgiven as far as Celtic fans are concerned. Taken outside of the Ten in a Row narrative, however, it's not something that will set them back years. Because the groundwork is there for whoever takes over build on.
Eyes were rolled when Lennon attempted to take the positives from Celtic's Old Firm defeat at the weekend, but he was right. Celtic were genuinely impressive. They dominated the league leaders for the first hour, and would surely have won, had Nir Bitton's ludicrous decision-making not turned the game on its head.
The much improved performance was a symptom of the adaptability Lennon has found somewhere deep within himself. The emergence of David Turnbull and Ismaila Soro, previously peripheral figures, has turned Celtic into a team capable of winning again; they'd won six on the bounce, including a European tie and a cup final, heading into Saturday.
Lennon's failure to deliver Ten will cost him his job, whether now or in the summer, but the team he leaves behind shouldn't require a complete rebuild before it can compete again. A midfield core of the enigmatic Turnbull and the classy, commanding Soro, complemented by the experience and winning mentality of Callum McGregor, is as promising a foundation as you will find.
In their collection of strikers, there are goals somewhere. Odsonne Edouard and Leigh Griffiths are proven scorers at Premiership level, Patryk Klimala is raw but promising, and Albian Ajeti shouldn't be written off entirely just yet.
Jeremie Frimpong, though yet to settle into a best position, looks genuinely prodigious, while there is no shortage of other flair players. Ryan Christie's output remains strong despite a relatively poor season while Olivier Ntcham and Tom Rogic, currently out of favour, can unlock defences.
And that's before the injured James Forrest is reintroduced.
They need work at the back, undoubtedly, but their first choice central defensive partnership is as strong as they could realistically wish for. The formidable duo of Christopher Jullien and Kris Ajer have been limited by injury this season but they compliment each other's attributes well, and are individually far better than the Scottish Premiership.
To challenge Rangers next season they will need a new goalkeeper, unless Vasilis Barkas can prove he is worth his hefty price tag between now and the summer. They will need to strengthen at full-back too: other than Frimpong, Greg Taylor and Hatem Abd Elhamed are the only two owned by the club who stand a genuine chance of long-term involvement.
Further defensive depth will be required (Shane Duffy is not the answer, who'd have thought) as will a midfielder with leadership qualities, since Scott Brown is surely off.
With three or four good additions, however, there is a squad there that can immediately get back to winning, and in a more sustainable way than has been the case since Lennon took the reins for a second time.
Celtic got so used to being the best that they thought they'd never be beaten. But it was always going to happen eventually - it's just cruel that it's happened now, with such an historic landmark on the horizon.
Failing to make it over the finish line of finish lines, though, doesn't detract from the work that has been done to this point. A quadruple treble isn't nothing, but it will quickly lose meaning if the club choose to wallow in self-pity instead of fighting back.
It may not seem like it now, while wounds are fresh, but Celtic can use the second half of this season to their advantage. Ten is gone but that doesn't mean it's game over.
It's simply the end of a chapter, and provided the club hear the alarm bells ringing and wake up from their complacent slumber, the next one holds serious promise.