After a terrific 40 minutes of hockey on Tuesday night between the Bruins and Montreal Canadiens, the sellout Garden crowd was settling in for what they hoped would be another great chapter in this storied rivalry in the third period.

What they witnessed instead was a massive Bruin meltdown.

Amidst a flurry of penalties, the B’s gave up four third-period goals — two coming on 5-on-3s — and lost to the Habs, 6-2, for their fourth straight loss (0-3-1).

Not a great way to head into the Christmas break.

On Monday, David Pastrnak had called this game a “must-win” after a bad loss against Ottawa. A win didn’t happen.

“It had everything. Guys winning fights,  guys laying their body out. At the end of the day, I have to be better. I’m not happy with my game, so this one’s on me,” said Pastrnak, who was minus-2.

The Habs broke a 2-2 tie at 7:04 of the third period on a flukish play. Nikita Zadorov’s stick snapped in his hands and created a turnover. Nick Suzuki broke in alone and Jeremy Swayman made a skate save but was in no position to stop the rebound attempt and Zach Bolduc scored.

Suzuki made minimal contact with Swayman and B’s coach Marco Sturm, who declined to challenge a waved-off Bruin goal at the end of the second because he felt Pastrnak’s stick was in Montreal goalie Jacob Fowler’s pad and it would be hard to overturn, decided to challenge this one.

It didn’t go his way, the goal was upheld and disaster ensued. The B’s were charged a minor and then found themselves down two men when Tanner Jeannot was whistled for crosschecking. It was a ticky-tack call (“A soft call,” said Sturm) but it didn’t matter. On the two-man advantage, Cole Caufield made a mind-boggling move to tuck it between Swayman’s legs at 8:43.

“All those challenges…you just never know. Looking back afterwards, could we have made a different call on one of them? Maybe. But those are split seconds and sometimes you have to make a decision right away. We trust everyone. There’s a lot of guys looking at those situations at once and we made that call. And today it didn’t happen,” said Sturm.

The Habs weren’t done. With Montreal still on a one-man advantage, Hampus Lindholm was called for tripping and, at 10:06, Suzuki scored.

Juraj Slafkovsky added another one and what was once a great game turned into a blowout in the end.

The start was so promising. After the morning skate, Sturm made a declaration.

“I know my guys will be ready,” said Sturm. “I don’t know if we’re going to win, but they’ll be ready.”

It didn’t take long to see those words were not hollow. Just like the first meeting between the two teams, there were fisticuffs off the opening draw – and it was good one. Jeannot locked up with Josh Anderson and the two big men staged a lengthy brawl that had the Garden crowd on its feet. Both landed some shots until Jeannot scored with a knockdown overhand right.

Midway through the period, Arber Xhekaj and Zadorov went at it. The appointment was made in the Bruins’ zone and Xhekaj skated to the center circle for dramatic effect. The Hab defenseman, who was quickly felled by Jeannot in the Nov. 15 game in Montreal, landed some good shots on Zadorov before the Bruin defenseman wrestled him to the ice.

“After (Jeannot) fought, guys on the bench said it was the loudest they’d heard the building,” said Alex Steeves.

With the tone appropriately set, the teams got into some hockey. It was the Habs who drew first blood on the scoreboard at 11:08.

It started with a bad turnover by Mason Lohrei (a team-high four giveaways) in his own zone that created chaos. Eventually, Sammy Blais banked one off the back of Swayman’s head from behind the goal line to give the Habs a 1-0 lead.

But Lohrei redeemed himself immediately and the B’s took a 2-1 lead into the first intermission. After the Habs turned it over in their own zone, Lohrei made a nice move at the left point to elude a Montreal defender and took it in deep, making a great pass to Marat Khusnutdinov for the young Russian’s third of the year at 12:25.

Fowler, the former Boston College star, made some excellent saves in the first and he was helped out by his post on a Morgan Geekie power-play shot. The B’s held a 17-8 shot advantage in the first. It looked like the B‘s might have to settle for a tie after 20 minutes.

But with just 18 seconds left in the first, Steeves gave the B’s the lead. Operating on their third power play, Steeves beat Fowler with a shortside wrister form the right circle, his eighth of the season.

It didn’t last long. Early in the second, Zadorov tried to tap a loose puck up to David Pastrnak in the neutral zone from his blue line. But Pastrnak had already headed up ice and the puck skittered into no-man’s land. Highly skilled rookie Ivan Demidov was only too happy to pounce. He went in on Swayman on a clean breakaway and beat him on a backhand move to even the game at 1:58 of the second.

The B’s were forced to kill off a double-minor to Steeves for high-sticking on a careless swing of his twig with 5:23 left in the period. What made it even more difficult was Zadorov could not get on the ice because of an apparent equipment issue. But the B’s managed to kill it off.

They also thought they retaken the lead wit 4.7 seconds left in the second after Geekie set up Elias Lindholm but it was ruled that Pastrnak had interfered with Fowler and immediately waved off. There wasn’t much contact there but the stick was in there and, considering the call on the ice was no goal, Sturm did not challenge.

It seemed OK at the time. The game was tied and they had a period to win to have a happy Christmas break. But it all fell apart in spectacular fashion.

“It’s disappointing to lose four in a row at home. It is disappointing,” said Zadorov, who was throwing punches at Canadiens till the end. “It’s not what we want to show our fans. We’ve got three days break and we’ve got to get it together. We’re still there. There are tight standings in the East. We’ve got to go to Buffalo right now and then we have the west Canada trip. We’ve got to come up with some points if we want to be a playoff team.”