BOSTON -- For 21 minutes of Game 1 of the Stanley Cup Final, the St. Louis Blues looked like the composed, confident team that won the Western Conference. They took a 2-0 lead by forcing turnovers with their punishing forechecks, and rookie goalie Jordan Binnington turned every Boston Bruins chance aside.

But from the moment Bruins defenseman Connor Clifton scored at 2:16 of the second period through Boston's celebration of its 4-2 victory, the Blues weren't the Blues. They were sloppy. They were undisciplined. And, in a rarity over their past several postseason games, they were overmatched.

"They're a good team, and they're going to force us into bad situations and things like that. But we need more than we gave tonight," said Blues coach Craig Berube.

The Bruins had a 33-15 advantage in shot attempts at 5-on-5 through the final two periods. They attacked the Blues' zone in waves. They prevented the Blues from generating anything close to established offensive zone time after Vladimir Tarasenko's goal a minute into the second period, giving Bruins goalie Tuukka Rask an easy night.

"What did we have? Fifteen, 16 shots? That's not enough," said Blues center Brayden Schenn, low-balling the total of 20 shots for St. Louis. "He's a world-class goaltender. We've got to not only shoot more pucks, we didn't get enough traffic around him. Tips, screens, we didn't make it very hard on him tonight."

Part of that struggle was not getting the puck deep enough to generate chances.

"They're a forecheck team. That's how they establish territory," said Bruins coach Bruce Cassidy. "I think we just know how to check. I think we've done it to Carolina, Toronto, Columbus. We know how to play winning hockey when we need to and limit the other team's chances."

But the great undoing for the Blues in Game 1 was their frequent trips to the penalty box: five of them for a total of 9:37 of penalty kill time. They entered the game talking about discipline in light of the Bruins' stellar power play, which clicked at a 34 percent success rate through three rounds, the best since the 1980-81 New York Islanders. They ended up playing with fire and then getting burned by an unassisted power-play goal by Charlie McAvoy in the second period to completely erase their lead.

It wasn't just that the Bruins converted, it's that killing those penalties left the Blues' attack disjointed.

"We went to the box too much. We lost our composure a little bit," said forward David Perron. "When you get into penalty trouble, a lot of guys sit on the bench, and it's tough to get going. That's what happened a little bit."

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Meanwhile, the Bruins were the more disciplined team, with just two penalties called on them.

"We stayed out of the box. I think that helps limit the chance attempts. They're spending energy killing penalties, so that takes away some of their offense," said Cassidy.

The silver lining for the Blues is they still have a chance for a split in Wednesday's Game 2, and they've shown a propensity for bouncing back from these kinds of efforts; Binnington, in particular, has led those efforts.

"It's not always going to be perfect," said Binnington, who is 5-2 after losses in the playoffs. "It's Game 1. It's a seven-game series. We're going to regroup and come back at 'em."