An autumn that featured an unlikely title tilt had lurched into a bleak winter, before a final-day defeat left Aberdeen fifth in the Scottish Premiership.
They headed into this showpiece occasion on the back of four consecutive defeats and had lost their last four meetings with Celtic 5-1, 5-1, 1-0 and 6-0.
With that in mind, Thelin set up with a back three, intent on denying Celtic space and opportunity to inflict such damage. The plan worked.
Celtic may have had the majority of the ball and territorial advantage, but they struggled to create clear chances, with Greg Taylor closest with a shot over the bar. For all their defensive solidity, Aberdeen threatened even less.
The Scottish champions usually find a way, though, and enjoyed some good fortune from a set-piece. Arne Engels' corner evaded Cameron Carter-Vickers but flicked off Dorrington, and clipped the inside of the post before nestling in the net.
It was cruel, as Celtic had the lead still without a direct effort on target, but it felt inevitable given the constant direction of travel.
The question was whether Aberdeen would open up with nothing to lose. They tried, but to little effect other than fleeting moments.
What proved was crucial was that they stood firm and didn't concede further. Engels came closest with a low shot that crashed off a post.
But the second goal didn't arrive and Aberdeen capitalised, again with a stroke of fortune. Morris scampered into space on the right and flashed over a dangerous ball that Schmeichel should have dealt with.
But the veteran Dane allowed the ball to slip under him and into the net.
In injury time, it was bedlam. Daizen Maeda raced clear and looked odds-on to seal his own incredible season with a dramatic winner, but Mitov stayed big to divert.
Then Celtic substitute Jeffrey Schlupp smashed the ball against the crossbar as the game swung back and forth like a basketball match in its final seconds.
But the greatest drama was to come, and when McGregor missed the opening spot-kick, Aberdeen would not look back as they went on to earn one of the most famous wins in their history.