Plekanec snapped a wrist shot from the right circle that beat goalie James Reimer to the short side for his 18th goal of the season, handing the Maple Leafs their fourth consecutive loss. It came after the Canadiens had failed to hold leads of 2-0 and 3-2.
"That was definitely a test of character," Montreal goalie Carey Price said. "When a team scores in the third period to tie it up when they're at home, you're on the road, they grab a lot of momentum. And being able to score at the end of that -- I don't think it was a power-play goal, time had expired, but anyway, being able to grab the lead and then hold it with a good team effort like that is, I think, rewarding."
Montreal went on a power play after Maple Leafs forward James van Riemsdyk was called for goalie interference at 9:14. Plekanec's goal came two seconds after the penalty expired, leaving Toronto coach Randy Carlyle frustrated.
"We clawed back into the hockey game and then we take a penalty early in the third and they score a [goal just as the power play expired]," Carlyle said. "It was the difference in the hockey game, and the margin of error now in these games is so close that one bounce or one misplay or one unfortunate mistake cost us points."
Toronto's Nazem Kadri tied the game at 3-3 with a power-play goal 2:49 into the third period. Joffrey Lupul found Kadri in front of the net for a quick shot that squeezed between the post and Price's pad for his 18th goal of the season.
The Maple Leafs had chances in the final minutes but couldn't get another puck past Price.
"They're close. They're close games," Maple Leafs captain Dion Phaneuf said. "We seem to be coming from behind. We've talked about our starts, but tonight we fought back. We knew it was going to be a good game. Both teams need the points; it's a crucial time of year. They found a way to get one more than we did. Tomorrow's a new day and we move forward."
Price finished with 33 saves, one more then Reimer, on a night when each team had 36 shots and plenty of scoring chances.
"Carey gave us a huge chance to win this hockey game," Canadiens coach Michel Therrien said. "He made some key saves, and we could tell that our team's playing with a lot of confidence when he's there."
The victory moved the Canadiens (39-26-7) within one point of the Tampa Bay Lightning in the race for second place in the Atlantic Division. Toronto (36-28-8), which visits the New Jersey Devils on Sunday, owns the first wild-card position in the Eastern Conference for the Stanley Cup Playoffs.
"It was a big game," Price said. "Obviously everybody's trying to collect points and make it to the playoffs, and when you have a divisional team like that, it's definitely crucial to try and get those wins."
The teams combined for five goals in the first period, followed by a scoreless second period.
"Not too often two defensive teams like that do you see 3-2 after that," Price said. "It definitely feels good to shut it down after that."
Rene Bourque made it 2-0 when he scored 1:05 after Pacioretty's goal. Bourque broke in against Reimer and sent a wrist shot over the goaltender's glove for his ninth goal of the season. It was Bourque's first game since March 8 after being a healthy scratch for the past five games.
"He really responded well which is what you want from a player out there, the way that he responded," Therrien said about Bourque. "He was hungry. He went to the net and made some good plays. He was solid. I liked the contributions of that line with Daniel Briere and Brian Gionta. They gave us some big goals."
Lupul got the Maple Leafs on the board at 11:02. Mason Raymond slipped a pass from the half-wall to a wide-open Lupul, who ripped a one-timer from the high slot past Price for his 20th goal of the season.
Tyler Bozak made it 2-2 with 1:57 left in the period when he put a loose puck behind Price, but Montreal regained the lead 64 seconds later when Brian Gionta's shot deflected off the stick of Maple Leafs defenseman Tim Gleason and past Reimer for his 15th goal of the season.
"Obviously we were behind the 8-ball a little bit early on giving up two goals," Raymond said. "I like the way we battled back, but turnovers killed us a bit and that's tough. That loss definitely stinks."
Bozak's goal was the first power-play goal allowed by the Canadiens since March 6; they had killed off 25 consecutive penalties.
Maple Leafs center Dave Bolland played in his first game since suffering a cut left ankle tendon Nov. 2. After missing 56 games, Bolland played 9:01; he had no points and one shot on goal.
"I felt great," Bolland said. "I felt pretty good with everything that was going on out there in all situations."