Derry 1-15 Dublin 2-12: The cat is well and truly out of the bag when you beat the reigning National League and All-Ireland champions by six points, and from now on Derry will be seen as a serious threat.
What happens down the line is a different matter of course, but one has to be struck by the fact that Donegal were beaten by Down on the day their Ulster Championship opponents Derry accounted for Dublin, whatever the personnel available.
Derry players and management got into a huddle afterwards and when it broke up, a sizeable Celtic Park crowd, unwilling to let go of the moment, clapped off their heroes.
If there was a moment that Derry re-entered the conversation, this was it.
Though with 20 minutes left, it seemed inevitable that Dublin would progress through the gears and grind Derry down. They were three points down at a stage when both Declan Mullan and Ciaran McFaul had a chance to halt the progress of Dublin coming out of defence and they didn't even contribute as much as a token tackle.
From that point, man of the match Mark Lynch hit three of the next four scores – two spectacular efforts from play raising the collective spirit and inspiring a late push.
Lynch was modest about his own performance. Some of his second half scores belonged in a highlight reel of marquee forwards yet when asked about them, passed off 45 metre kicks as being "laid on a plate" for him.
Brian McIver (below) was rather more effusive, saying Lynch was "absolutely immense", before spreading the praise.
"Patsy Bradley was absolutely brilliant, some great catches, serious hit there with 10 minutes to go. Fergal Doherty, Kevin Johnston was excellent, wee Enda Lynn ... Ciaran McFaul I thought had a brilliant 40 minutes," he said.
"And the lads coming in, we were a bit like the rugby yesterday, fresh legs coming in and didn't look out of place."
The emotion of the game drained everyone, packed as it was with incident. Derry were level on 0-5 each after the first 20 minutes, one point from Lynch where he held off Davy Byrne like a child, a pointer to their intent.
They also benefitted from some super goalkeeping by Thomas Mallon.
The solitary goal came on the half hour. An attack was floundering and it took desperate grabs from Sean Leo McGoldrick and Kevin Johnston to get the ball into Lynch's hands, who potted it delicately beyond Stephen Cluxton.
Just before the break Cailean O'Boyle was hauled down in the square by Sean George but referee Fergal Kelly waved away appeals for a penalty and a black card for George, much to the disgust of the home crowd and Derry trainer Paddy Tally who ran on to the pitch to remonstrate.
From then on, it was backs to the wall stuff in the Bogside.
Dublin's finishing was off. Thomas Brady hit the base of the post in the first play of the second half and while Mark Craig's fatigue may have contributed to picking up a yellow, black and red card within three minutes, the possible benefits were negated by Philly McMahon's red card for an off-the-ball strike on Lynch.
From that point, Derry made the crucial plays as Dublin faded and allowed the margin to grow.
Dublin manager Jim Gavin looked far from bothered. He had rested eight of the under-21 panel for this game, with some stellar talents such as Jack McCaffrey, Paul Mannion and the other Davy Byrne held back.
Add to that the names of Bernard Brogan, Rory O'Carroll, Ger Brennan and Diarmuid Connolly, and you can see why he made peace with the result fairly easy. He also got to re-introduce Alan Brogan after his lengthy spell out with Osteitis pubis, and he hit 0-6 of the Dubs' tally.
"I think we'll learn a good bit from today," reflected Gavin.
"It's a great experience for the players, to come up to Derry this morning. And to play against a very good team and to experience that sort of atmosphere. We're obviously disappointed with the loss but it was a great game to be involved in."
McIver was more bullish in his final assessment.
"Now we know we are able to play football with the best in the country. But, this is only the start of this side."
DERRY: T Mallon; M Craig, C McKaigue, D McBride; K Johnston, G O'Kane, SL McGoldrick; F Doherty 0-2, P Bradley; C McFaul 0-2, M Lynch (Capt) 1-8, 4f, E Lynn; B Herron 0-2, D Mullan 0-1, C O'Boyle 0-1. Subs: R Bell for O'Boyle (39m), A Devlin for McFaul, C McGoldrick for Bell (55m), A McAlynn for Doherty (67m). Yellow cards: Craig (56m), O'Boyle (35m), McKaigue (46m), McFaul (53m). Black cards: Craig (59m). Red cards: Craig (59m)
DUBLIN: S Cluxton (Capt); S George, P McMahon 0-1, M Fitzsimons; D Daly, D Nelson, J Cooper; C O'Sullivan, MD Macauley 0-1; D Byrne, A Brogan 0-6, 1f, B Cullen; P Hudson 0-2, 2f, P Andrews 0-2, E O'Gara. Subs: K Nolan, T Brady, J Whelan 0-1, 1f, for George, Cullen and Hudson (half-time), N Devereux for Nelson (43m), C Reddin for Byrne (53m), D O'Mahony for O'Sullivan (67m). Yellow cards: Daly (26m). Black cards: 0. Red cards: McMahon (60m).
Referee: Fergal Kelly (Longford).