Two goals to the good against the Diamonds, and with the advantage of a first-leg 1-0 win, Montrose only had to close out the second 45 to set-up a final with Queen’s Park.
Instead the hosts came roaring back in to the tie, inspired by striker Callum Smith.
Not only did Airdrie level the tie, they would end up taking the match to extra-time where they closed out the win to leave Stewart Petrie’s side devasted.
The hosts started brightly and had a couple of early penalty claims snubbed, the loudest shout coming when Smith took a tumble under pressure.
Ref Alan Newlands, though, was having none of it.
Airdrie continued to pile on the pressure and Allan Fleming had to look lively to keep out Calum Gallagher’s drive.
But it was to be Montrose who grabbed the game’s first goal and give themselves a 2-0 lead in the tie when Cammy Ballantyne’s cross was nodded home by Cammy F Ballantyne.
Things would get even better for Stewart Petrie’s side when they went into a 2-0 lead with 26 minutes gone.
Cammy F Ballantyne was again the scorer, this time converting a Lewis Milne cross.
Smith had a chance to reduce the deficit but his header from eight yards drifted high and wide.
Airdrie were unlucky not to get their name on the scoresheet before the break and only a Cammy Ballantyne goal-line block denied Craig Watson.
The hosts knew they needed a goal to try and make Mo sweat and Dylan Easton thought he had done just that only to see his long range drive crash back off the crossbar.
They got the goal they craved on the very next attack, though, when Smith cut inside on the edge of the box and found the bottom corner.
Montrose showed some excellent composure and refused to buckle under the pressure of an Airdrie fightback. They restored their two goal cushion when Craig Johnston scampered clear and found Milne.
The Mo forward picked his spot and beat Max Currie.
Airdrie refused to lie down and hit back almost straight away through Smith once again.
The goal gave the home side a massive boost as they sensed the tie wasn’t quite yet dead and buried.
Smith completed his side’s comeback when he netted his third to level the score on the day and bring the Diamonds to within a goal of their visitors overall.
Play was only going in one direction as Airdrie went for broke.
Mo were gripping on to their lead by their fingernails but let it slip in the closing seconds when Jon Afolabi’s delivery was turned in by Calum Gallagher.
The momentum was completely with Airdrie by this point and they went into extra-time with a spring in their step.
They took the lead for the first time in the tie shortly before the end of the first half when Afolabi shifted the ball well from underneath his feet and got a shot away which beat Fleming.
Gabby McGill wrapped the tie up in the closing stages of extra-time to set up a play-off final date with Queen’s Park.
Martin Rennie reduced the deficit but it was all too little too late for Montrose.
Mo had given their absolute everything but had come up just short again at the line.