An 84th-minute goal by Dominic Oduro was enough to secure the Impact's passage to the Amway Canadian Championship final in a rollercoaster game against Toronto FC on Wednesday.
Toronto FC won 3-2 on the night but the aggregate was tied 3-3, thanks to a 1-0 triumph in last week's semifinal first leg, so the Impact advanced on the away goals rule.
Montreal will meet the winner of the Vancouver Whitecaps-FC Edmonton semifinal to determine who will represent Canada's in next year's CONCACAF Champions League.
The dagger in the heart came when Oduro, a former Toronto player, and his mohawk rose majestically to beat Damien Perquis and head in an Eric Miller cross.
Montreal (0-3-2) has yet to win in MLS play this season. But the Impact have demonstrated their resilience in cup play and took advantage of slack Toronto defending on both goals.
"It shows we don't give up at the end of the day," said Oduro. "We fight hard, until the whistle is blown ... Hopefully we can build on that character going forward."
Up until Oduro's strike, it had looked like two goals in three minutes would lift Toronto past the Impact, with designated players Jozy Altidore, Michael Bradley and Sebastian Giovinco running the show.
Goals by Benoit Cheyrou and Giovinco in the 56th and 58th minutes put Toronto ahead.
The Impact were down a man at the time as trainers worked on Maxime Tissot, who had crashed heavily into the end boards and was receiving treatment off the field.
"Those five minutes changed the game," said Montreal coach Frank Klopas, who said he was unable to get a quick answer on Tissot's condition from the medical staff so he couldn't make changes.
Tissot stayed in the game and Klopas said he seemed OK afterwards.
"I keep asking him what the score was and he seems to remember," said the coach, who seemed uncomfortable at the confusion over his injured player.
Altidore, who set up both second-half goals with help from Bradley, opened the scoring for the home side.
Kenny Cooper also scored for Montreal, whose first-leg goal came from Jack McInerney.
Toronto manager Greg Vanney had promised a more aggressive approach after a 2-1 weekend loss to visiting Houston and delivered with a high-powered lineup and attacking formation.
Vanney, who rested all but three regulars in Montreal, rolled out eight of his starters from the weekend. The almost full-strength side featured all three designated players as well as European imports Cheyrou and Perquis.
In contrast, Klopas stuck with just four starters from the weekend and kept DP Ignacio Piatti on the bench until the 68th minute.
Altidore opened the scoring in the 22nd minute, on a beautiful one-two with Giovinco. Montreal replied three minutes later on a corner that was flicked over to an unmarked Calum Mallace, whose volley was flicked in by Cooper from in front. The away goal meant Toronto had to score two more to overturn the aggregate deficit.
It was Cooper's first goal for the Impact but ninth in 11 games against Toronto.
For Toronto (3-5-0 in league play), the loss was a kick in the teeth.
And for the second game in a row, the five-foot-four Giovinco took a beating from the opposition and was clearly unhappy at the lack of protection from referee Mathieu Bourdeau.
One uncalled foul left Giovinco bleeding, said Vanney.
"I'm totally irritated at the fact that the referees don't do a job, which is to protect some of the key guys on the field," he said.
"This league has talked about it since Day 1," he added. "And they've got to follow through with it."
The game drew 21,069 to the newly renovated BMO Field, down from 30,226 at Sunday's home opener.
That contest was marred by fans abusing a TV reporter, a story that went viral when the reporter confronted those responsible.
On Wednesday, MLSE said it was reaching out to TV assignment desks about plans for live hits at the stadium with security to be allocated accordingly. It also said it had identified four fans and banned them for at least a year.
Follow @NeilMDavidson on TwitterSuggest a correction