Calum Gallagher was Airdrie’s last action hero as the Diamonds booked a place in the Championship play-off final in unbelievably dramatic fashion.

It took extra-time to separate two sides who have been neck and neck over the season, with a 1-1 draw at Cove’s Balmoral Stadium on Saturday meaning the semi-final second leg was perfectly poised.

Dean Ritchie came into the Diamonds line-up in place of Kyle Connell as manager Ian Murray aimed to strengthen the midfield and cut the supply to the Cove Rangers front two.

Paul Hartley’s Toonsers went with the same line-up which had come from behind to earn a draw in Saturday’s first leg.

Airdrie’s gameplan suffered an early disruption when Paul Paton pulled up after only six minutes. The experienced midfielder was unable to continue and Connell replaced him.

As the sides sized each other up, early chances were at a premium. Veteran Cove marksman Rory McAllister found space to get a shot away from the edge of the box on nine minutes, but his effort was quickly charged down.

Cove keeper Stuart McKenzie got his first involvement from Connell’s long throw two minutes later, touching the ball away from danger before smothering it.

The deadlock was broken on the 15th minute, and it was the visitors who opened the scoring through captain Mitch Megginson, who rose to guide a Connor Scully free-kick into the top corner of Max Currie’s goal.

It took only four minutes for the Diamonds to reply, Connell lining up a long throw on the left and intelligently going short to Kyle Turner, who returned the pass. Connell’s cross was allowed to bounce across the area, and Calum Gallagher threw himself full-length to equalise with a diving header at the far post.

Callum Fordyce was joining the Airdrie attack whenever possible, and found space to knock in a downward header on 21 minutes, but McKenzie was well placed to catch.

Midway through the first half, Cove goalscorer Megginson went down injured and had to be replaced by Connor Smith, the brother of former Diamonds loanee Callum.

McAllister sent a free-kick over the bar on 25 minutes before the Cove players were claiming for handball two minutes later, but Turner knew little about the contact as a clearance struck him from behind.

Currie punched out a Fraser Fyvie corner as the visitors continued to threaten, and the goalkeeper was at his best to tip over a powerful strike from the same player on 32 minutes.

Chances for the home side were few and far between as half-time approached, but Ross Graham did well to poke the ball back to McKenzie as Connell threatened with ten minutes of the half remaining.

Gallagher saw a low cross hacked away before Graham again cleared under pressure from Connell.

The final action of the first half saw Cove substitute Smith do well to engineer some space, but a well timed tackle from Ritchie averted the danger.

The second half saw Airdrie captain Sean Crighton replacing Josh Kerr, who had gone down injured on the stroke of half-time.

Some early Diamonds pressure saw Walker and Gallagher combining down the right flank before Kieran Ngwenya had to deal with a Fordyce cross.

The visitors began to control possession but the Diamonds were defending well and Currie was showing safe hands when called upon, holding a Scully free-kick ten minutes after the restart.

Smith was finding good positions for Cove, but his shot was off-target on 68 minutes.

Crighton nodded a Connell throw just wide a minute later before a Connell dummy was hoofed clear after good work by Diamonds sub Thomas Robert.

Smith and Crighton exchanged chances as the game moved into the final ten minutes, with Connell having to be withdrawn following a head knock. Jack McKay replaced the stricken striker with five minutes to play.

Disaster struck for the Diamonds in the second minute of stoppage time as a tackle on the edge of the Airdrie box broke kindly for Blair Yule who fed Rory McAllister, the striker taking a touch and stroking the ball past Currie.

It was a finish typical of the former Peterhead frontman, who wasn’t alone in thinking he had won the game for the Aberdeen side.

The Diamonds had other ideas though, Fordyce curling an effort straight at McKenzie as the clock ticked down.

With 94 minutes played, a long ball into the box from Currie saw his opposite number race forward to claim, only to drop the ball at the feet of Carrick. The Diamonds forward showed great composure to knock the ball into the path of Jack McKay, who calmly struck home on the half-volley despite the despairing dives of two Cove defenders on the line.

The goal was testament to Airdrie’s determination throughout the season, and gave the Diamonds a huge lift going in to extra time.

The first period was goalless, but both sides looked to be going all out for the win, Smith seeing a shot deflected behind before Fordyce had a great chance to net, but his downward header had too much on it and bounced up over the bar.

Craig Thomson, who had replaced Dale Carrick on 99 minutes, added a new dimension to the Airdrie attack, and his 101st-minute cross was booted clear at the front post.

Currie saved well with his feet before claiming a Fyvie cross before the half-time whistle.

Scully was perhaps lucky to escape with a booking after a lunge on Fordyce early in the second period, but the Diamonds responded in the best possible fashion with a thrusting run from Thomson which took the midfield man into the box to lash in a low cross which Gallagher volleyed home.

With Cove throwing everything forward, Jack McKay could have tied it up on 118 minutes as Robert played him in on the break, but a heavy touch allowed McKenzie to gather.

A late red card for Paul Hartley in the Cove dug-out signalled the end of the visitors’ chances as the Diamonds held the ball in the corner until referee Colin Steven sounded the final whistle.

A two-legged play-off final against Morton now awaits the Diamonds, who rushed to the corner of the pitch to applaud the hardy bunch of fans watching the game from Petersburn Road.