St Mirren came from behind to beat Welsh Premier League champions The New Saints and book a Scottish Challenge Cup final date with Dundee United.
Ryan Brobbel swept a dipping shot over St Mirren keeper Billy O'Brien to give the visitors the lead before half-time.
But in a dramatic five-minute spell, Stephen McGinn rifled in a 25-yarder before TNS' Steven Saunders saw red and Stevie Mallan curled in the free-kick.
John Sutton added a third before Rory Loy grabbed the fourth in injury time.
The all-Championship final, 30 years on from St Mirren beating Dundee United in the 1987 Scottish Cup final, will take place on the weekend of 25/26 March, at a venue yet to be confirmed.
United won 3-2 at Queen of the South in Saturday's first semi-final.
TNS were on a run of just one defeat in 31 matches and had earlier knocked out Forfar and Livingston in the competition.
Chances were few and far between in the early stages, with only Mallan's 15th-minute free-kick from 20 yards giving the home fans anything to cheer about.
Brobbel gave a hint of what was to come after 35 minutes with a driving run and shot which produced a good save from Billy O'Brien, before Wes Fletcher grazed the outside of the post with a header from the resulting corner.
But four minutes from the break, Brobbel produced a goal of sheer class when he ran from midfield, looked up and curled a terrific strike from 22 yards into the top corner past O'Brien.
Saints boss Jack Ross sent on Lewis Morgan for Craig Storrie at half-time and initially TNS continued with the same tactics of allowing the hosts plenty of the ball and hitting on the break.
But the momentum changed dramatically in a five-minute spell around the hour mark.
First McGinn crashed a brilliant 25-yard strike high past Harrison for the equaliser, then former Motherwell defender Saunders dived into a challenge with the Saints captain to leave referee Keith Kennedy with no option but to send him off.
From the resultant free-kick, Mallan again underlined his quality by placing his fifth goal of the season into the bottom-left corner to put the home side in front.
Confidence was now oozing through St Mirren, and despite Sutton seeing a penalty saved by Harrison after 79 minutes, the veteran striker made no mistake a minute later as he tucked home his 11th goal of the season after Kyle Magennis had set him up.
Low then set the seal on Saints' victory, knocking in Mallan's injury-time free-kick.
St Mirren boss Jack Ross: "It is all about the result at this stage of the competition, but we made it hard for ourselves after the performance in the first half. We were dreadful and I said that to the players at half-time.
"But in the second half we were terrific. Their reaction was great, our cup form is well documented, so different from our league form, but we need to win more league games.
"We are starting to build a little bit of momentum and we go into a massive game at Ayr next week in good spirits now having reached a cup final. Maybe the group needs results like this to give them the confidence they need."
New Saints boss Craig Harrison: "We are disappointed. We were the better team in the first half and needed to keep them out for the first 20 minutes of the second half, but didn't do it.
"The sending-off was right and it was the turning point. You just can't tackle like that, but that is the way it goes and they scored from the free-kick to give them a huge boost.
"It's a great feat for us to get to where we got to in this competition and I am proud of the players. I think if we look back, we would like to think we have done our club and the Welsh Premier League proud."
- 44EckersleySubstituted forIrvineat 90'minutes
- 20StorieSubstituted forMorganat 45'minutes
- 22McGinnSubstituted forFjeldeat 87'minutes
The New Saints
- 2SpenderBooked at 90mins
- 15SaundersBooked at 64mins
- 14MullanBooked at 46mins
- 8BrobbelSubstituted forSeargeantat 68'minutes
- 21CieslewiczSubstituted forQuigleyat 68'minutes
- 10FletcherSubstituted forDraperat 81'minutes
- Keith Kennedy
- Shots on Target