The starting point is worth repeating. In the autumn of 2010, Jim McGuinness took charge of a Donegal team who had been beaten to the point of perpetual dejection by Armagh in Crossmaglen.
He brought the cream of an under-21 team who had lost an All-Ireland final to Jim Gavin's Dublin, including Mark McHugh, a tireless playmaker, and Michael Murphy, who was emerging as probably the most powerful and fearsome attacking player in the game.
But the general wisdom was that Donegal might occasionally run with the serious teams but they would never seriously threaten them. Yesterday, as his team pulled off a victory for the ages, the usual question travelled around Croke Park: how had McGuinness managed to do this?
One answer is, of course, that McGuinness cottoned on to something early that is still dawning on some observers. He knew that in the McGee brothers, Karl Lacey, Anthony Thompson and Christy Toye, he had players of depthless potential.
And he knew that the younger generation were brimful with ambition. The genius lay in fusing the two elements together and then devising a game plan which is unlike anything witnessed in Gaelic football before. Yesterday was the most glorious proof of that.
But even afterwards, McGuinness was still in thrall to the machinations of the game, returning to the earlier period when Dublin were swift and confident in their movements and thrilling in their score taking.
They pressed up aggressively on the visitors, Rory O'Carroll flapping his hands in front of Big Neil Gallagher until the Glenswilly man finally snapped and drew a yellow card. But Donegal looked caught in the web and McGuinness explained the dilemma.
“ . . . And if you kick it in long, their full-backs are so tenacious they’re getting out in front and getting a hand on it. The challenge is to be able to get out in front with the ball and keep it away from them because they’re pressing you so high up the pitch.
“There were a number of players turned over resulting in scores and that’s a very difficult thing. Then when you get to the halfway line you have to decide do you want to run it or do you want to kick it. It’s very easy to kick it when it’s 2v2 or 3v3 but they’re excellent full-backs.
“It’s marrying all that and giving them that information and trying to work your way through it intelligently. Early in the game we didn’t look slick and we didn’t look composed but, the longer the game went on, I think we relaxed within the game plan and reverted back to what we have been doing over the last three weeks and settled into the game.”
Donegal are the forensic in their dedication but succumbing to an early Dublin scoring blitz had not been part of the masterplan. Trailing by 0-7 to 0-2 after 20 minutes was something that the players had to respond to.
“No, that was something that they had to think their way through. We were hoping that we were going to be a lot more compact in the early stages because the last thing you want is for Dublin to be rampaging through you and kicking long-range points.
"Every time they shot, it was over the bar. The quality, Paul Flynn in particular was phenomenal in the first 20 minutes of the game. We had to try to re-evaluate what we were doing and how we were doing it and our defensive structure.
“I think once we started getting a couple of turnovers and then on the back of getting the turnovers we started pushing on with a bit more intent, it was a bit wee bit too lethargic and our decision making was off because we needed energy, we needed pace, we needed men with a bit of intent. When we got that the game started to open up for us and that fuelled confidence.”
In came Toye. Donegal began to run with more purpose and confidence. They did what no team had managed to do against Dublin: they thought their way into the game. A 1-4 blitz before half-time turned all perceived wisdom on its head. Michael Murphy kept Dublin guessing, now on the edge of midfield, now loitering on the square.
Patrick McBrearty, held in reserve, thumped two huge second-half points. Colm McFadden fell into old habits, finishing with 1-3.
Nobody had seen it coming. A year ago, Donegal had looked a hollowed shell as Mayo ate them without salt in Croke Park. McGuinness insisted then and later that injury and circumstance had prevented them from preparing properly. Nobody listened.
He changed his backroom team, bringing in former team-mates and moved on. When Mark McHugh left the panel, it was offered as further proof that things had soured.
Again, the truth was both more complicated and more simple: the older McHugh just needed a break and in came his younger brother to play a similar role. On they rolled, full of quiet belief and limitless courage. Did they know this performance was in them?
“I don’t know, you’d have to ask the players that but I believe that every game is winnable if you get everything right and you’ve got the right attitude. Every single county in Ireland has 15 good footballers and if they’re all fully, fully focused and they’ve got the heart and determination to play for their county and there’s a good game plan there, you’re not going to be far away.
“We were hoping going into the game today that the honesty would be in the group, that we would deliver a game plan and we still didn’t know if we were good enough to win the match or not because of the challenge that was presented to us.
“Had we lost today by a point, I could have easily made peace with that because that’s what you want, you just want them to give it everything and deliver that performance and they did that.”