The Thrashers set a team record with 50 shots, more than double Montreal's 23.
"It was one of those nights again," Montreal goalie Jaroslav Halak said. "Many, many shots again. But we were able to win the game. That's all that matters."
Plekanec and Andrei Kostitsyn scored twice in the second period to give the Canadiens a 2-1 lead.
Michael Cammalleri assisted on both goals and Plekanec assisted on Kostitsyn's.
Ilya Kovalchuk 20th goal came with 17 seconds left in the first period to put Atlanta up 1-0. He beat Halak on a tip-in off a shot by Ron Hainsey. Atlanta outshot the Canadiens 18-3 in the period and 36-13 after two periods.
"We did exactly what we wanted to do the whole game. It was just one of those games," Atlanta coach John Anderson said. "The hockey gods weren't with us."
The Canadiens recalled LW Benoit Pouliot from Hamilton of the AHL. ... Atlanta GM Don Waddell spoke with Kovalchuk's agent, Jay Grossman, during the weekend about a long-term deal, but nothing came of it and they said they will talk again after the holidays. "We hit a little snag right now," said Waddell, who remained optimistic about keeping Kovalchuk, scheduled to become an unrestricted free agent after the season. ... The Thrashers open a 7-game road trip on Wednesday in Boston.
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ATLANTA - DECEMBER 21: Maxim Afinogenov #61 of the Atlanta Thrashers heads off the ice before the game against the...