Montreal moved two points ahead of Philadelphia and Boston in the tight Eastern Conference playoff race. The Flyers lost for the second straight night and have just one win in eight games (1-6-1).
The Flyers and Canadiens entered the night in a three-way tie with idle Boston for sixth place in the East. Philadelphia is only two points above the postseason cutoff.
Brian Boucher finished with 22 saves for Philadelphia, which has dropped nine of 11 overall. Boucher, one of six goalies used by the Flyers this season, has only one win in his last seven starts.
Plekanec gave Montreal the lead it protected all night when he took a blind centering pass from Andrei Kostitsyn and beat Boucher 9:25 into the game for his 24th goal.
Halak allowed five goals on 17 shots against the Flyers on Feb. 13, but was much stronger in the rematch. His best sequence came late in the second period when he made consecutive saves on James van Riemsdyk and Mike Richards. Just seconds later, van Riemsdyk skated in on a semi-breakaway and was denied again.
Halak did benefit from a fortunate bounce with 7 minutes left when a wrister by van Riemsdyk caromed off his shoulder and trickled just past the right post.
Philadelphia had a goal disallowed with 15:38 remaining because van Riemsdyk used a high stick to chop a floating puck into the net. The goal was immediately waved off on the ice, and the call was upheld after a video review.
NOTES: Flyers C Jeff Carter (foot) is scheduled to have an MRI on Wednesday. He is expected to return in time for the playoffs. The best-case scenario for injured Flyers G Michael Leighton is "at least three weeks," general manager Paul Holmgren said. Canadiens RW Sergei Kostitsyn missed the game because of a lower-body injury. Flyers D Chris Pronger played in his 1,100th NHL game. Flyers LW Dan Carcillo served the final game of his two-game suspension for high-sticking New Jersey's David Clarkson on Sunday.