Tomas Plekanec put one in each net.
The Canadiens forward made up for a first-period own goal by scoring the tiebreaker with 9:14 remaining to beat the Bruins 2-1 on Thursday night.
Carey Price stopped 29 shots for Montreal, but he took the blame for giving Boston an early lead when Plekanec won a faceoff and put it straight into his own net.
"I just Bill Bucknered it," Price said. "Just a mental lapse, for sure. I didn't want that to be the winning goal. Our guys did good to get it back and they bailed me out."
It was the teams' first meeting since a seven-game series in the first round of the playoffs that Boston won en route to its sixth Stanley Cup championship.
All of the old animosity was still there, with Brad Marchand and P.K. Subban the latest to brawl in an Original Six rivalry that's nearly a century old.
"When you are frustrated — and I know I was — it doesn't take much," said Subban, who played with Marchand in juniors.
"It wasn't Marchand who frustrated me. There was no animosity between us. … If I was in Montreal and saw him out to dinner, we'd probably hang out together."
After opening the season with just one win in its first eight games — its worst start in 70 years — Montreal fired assistant coach Perry Pearn. The Canadiens have now won two in a row, having beaten Philadelphia 5-1 on Wednesday night.
Tim Thomas made 33 saves for the defending Stanley Cup champions, who have lost three of their past four games and moved into last place in the Eastern Conference.
"I'll probably get nightmares on how we are playing right now," coach Claude Julien said. "Our inability to focus for 60 minutes is obvious and apparent."
Plekanec stumbled in the first period, winning a faceoff at the left circle but knocking it past Price into the Montreal net; Patrice Bergeron lost the faceoff but was credited with the goal. Erik Cole tied it for Montreal in the second, and then the Canadiens took the lead in the third when Plekanec's shot was blocked by Marchand right back to Plekanec, who beat Thomas on the second attempt.
But the game seemed at times to be a sideshow for the recurring brawl between Marchand and Subban, who squared off three times in the second period — with crowd-pleasing results.
The two matched up for a pair of holding penalties at 13:47 of the second. As soon as they came out of the box they found each other in the corner, but before any punches were thrown the linesmen separated them.
That only postponed the drama, because when they came out of the box they went right for each other again.
Subban, Marchand tangle
Circling each other at centre ice, and ignoring the action in the Canadiens' zone, the two players shed their gloves, their helmets, even their elbow pads as they put their dukes up and prepared to fight. When they finally closed in, after yet more bobbing and weaving, Subban unleashed a roundhouse right hand that missed; he slipped to the ice, and the fans laughed and jeered.
But the Canadiens' tough guy was quickly back on his skates, grabbing sweaters with Marchand as they traded punches. Subban also swung and missed a couple more times.
Finally, the two punched themselves out and just stopped; the linesmen ushered them off the ice for the rest of the period.
"Stuff happens in a game like this, when emotions are high," Marchand said.
Boston wound up with a power play because of a holding penalty on Josh Gorges, and the Bruins had two great chances. One of them was saved by defenceman Raphael Diaz, desperately sliding across the goal crease with Price out of position. Price lost his stick on the play, but he stopped a shot from the point between his pads to force a faceoff.