The Bruins notched their best characer win of the young season, scratching back from a two-goal deficit, climbing over some serious in-game adversity and pulling out a 3-2 overtime win over the Florida Panthers at the Garden on Monday.

Pavel Zacha scored with 1:24 left in overtime to lift the B’s to a victory that seemed improbable earlier in the night. Linus Ullmark, meanwhile, made 36 saves for the win.

After a brutal first period in which they went down 2-0, the B’s found their footing in the second with a goal and then Charlie McAvoy tied it up at 7:20 of the third period off a beautiful rush. Zacha made a nice touch pass to David Pastrnak, who had speed going through the neutral zone. Pastrnak sliced from left to right through the offensive zone and lifted a pass to the rushing McAvoy, who flipped his second in as many games past Sergei Bobrovsky.

But McAvoy put his team in a hole with a hit to the head on Oliver Ekman-Larson with the puck nowhere close to the area in the offensive zone, earning a match penalty and possibly a couple of games off. That gave the Panthers a five-minute power play and put the B’s down four defensemen after Matt Grzelcyk had left the game after the first period with an upper body injury.

The B’s and their tremendous penalty killers did their job and survived the major, pushing the game to OT.

It was the night before Halloween, but the Bruins experienced all their nightmarish flashbacks from last spring in the first period, when the Panthers won all the key battles at the blue lines, got their sticks in lanes to thwart scoring chances and utterly frustrated the B’s on breakouts.

The B’s incurred their first multi-goal deficit in the opening 20 minutes – and it could have been worse than the 2-0 score that it was. The Panthers dominated, outshooting the B’s 16-6 in the first.

The B’s had a glimmer of spark in the early going, but it quickly blew up in their faces.

Rookie Matt Poitras broke in on an odd-man rush and from the left wing fired a shot that Bobrovsky kicked out for a long rebound. With McAvoy pinched down at the right point, Sasha Barkov took off on a 2-on-1 with Sam Reinhart. Grzelcyk took Reinhart and gave Ullmark the shooter, but the goalie’s glove could not catch up to Barkov’s wrister at 6:13.

That seemed to throw the B’s into disarray, as they continued to cough up the puck all over the ice.

The Panthers doubled their lead at 15:08. Poitras lost the puck behind the B’s net and Barkov pounced on it. He circled the cage and found Reinhart out front for Reinhart’s eighth of the season.

To make maters worse, Grzelcyk suffered an upper body injury in the first and would not return to the game.

The B’s were much more assertive and it paid immediate dividends. On one shift, Kevin Shattenkirk dropped Anton Lundell with a forearm and Morgan Geekie deposited Nick Cousins into the Florida bench, creating the first buzz in the building.

Then, at 3:38, the B’s gave Florida a dose of their own medicine to draw to within a goal. Charlie Coyle kept a puck in at the blue line and fed a circling Jake DeBrusk. DeBrusk took it deep into the right wing and fed it out front for Brad Marchand, who just got his stick on it to nudge it past Bobrovsky, who’d come out of his net to face a potential DeBrusk shot. It was Marchand’s fifth of the year.

The B’s finally had some momentum, but it was thwarted by a bad call. Geekie was clearly knocked into Bobrovsky by defenseman Josh Mahura but he was sent off for goalie interference.

The B’s killed that off and, at the end of the kill, the Panthers lost Sam Bennett, who was making his season debut after suffering a lower body injury in the preseason. Bennett got tangled up with Hampus Lindholm and appeared to hurt his left knee, needing help to get off the ice.

Pastrnak earned the B’s a power play when he got dumped in front of the Florida net by Oliver Ekman-Larson. They could not capitalize.

Heading into the third, the B’s remained in striking distance, thanks to a terrific save on Evan Rodrigues’ one-timer in tight late in the second.