Plekanec opened the scoring early when he deflected Brian Smolinski's shot, that came off the right wing boards, past goalie Manny Fernandez. He made it 2-0 six minutes into the game with a power-play goal. Plekanec cut in on the left wing side and slid in a backhander.
The Canadiens took a three-goal lead in the second period when Plekanec wristed a rebound of Michael Ryder's shot high over Fernandez's shoulder during another power play.
But Montreal restored its three-goal lead minutes later when Chris Higgins put a weak wrist shot from the top of the slot past Fernandez. Higgins played for the first time in the preseason after sitting out four games.
He had a chance for a second goal when he was awarded a penalty shot near the end of the second period, but was stopped by Fernandez on a deke attempt.
The Bruins, who were outshot 33-17 through two periods, came out with a flurry in the third. They sliced the Canadiens' lead in half on a goal by Glen Murray that drew a large cheer from the crowd of 9,967 for the Nova Scotia native.
Phil Kessel scored against Halak with just 9 seconds left to make it 4-3.
Copyright by STATS LLC and The Associated Press
Arena: Metro Centre
Tomas Plekanec Assist: Bryan Smolinski, Mark Streit
Tomas Plekanec (power play) Assist: Roman Hamrlik
Tomas Plekanec (power play) Assist: Michael Ryder, Chris Higgins
Chuck Kobasew Assist: P.J. Axelsson, David Krejci
Chris Higgins Assist: Bryan Smolinski
Glen Murray (power play) Assist: Peter Schaefer, Patrice Bergeron