BOSTON — You could almost give them a mulligan for the first period.

We all knew the deal coming in to Game 1 of the Stanley Cup Final against St. Louis Monday night at TD Garden. It had been 11 excruciatingly long days since Boston had swept Carolina for the Eastern Conference crown, and prevailing wisdom was that it might take some time for them to get back into the speed and rhythm of playoff hockey.

That was true for 21 minutes.

The final 39 minutes? They belonged totally, completely and exclusively to the Boston Bruins.

Finally freeing themselves from the cobwebs that gathered during that week-and-a-half respite from actual competition, the Eastern Conference champions imposed their will on St. Louis in the second and third periods, scoring four unanswered goals to skate off with a 4-2 Game 1 triumph.

Down by two goals just a minute into the middle stanza, the Bruins turned the game around by first getting a quick strike from defenseman Connor Clifton before fellow blue liner Charlie McAvoy scored on his team’s fourth power play attempt. The latter strike tied the game at 2-2 and took away any momentum that the Missourians had carried to that point.

“We were able to join the rush in those situations and contribute offensively,” captain Zdeno Chara said of the Bruins’ defensive corps. “I think we do rely on those kind of situations at certain times of the game.”

Boston plastered rookie netminder Jordan Binnington in the second frame, putting 18 shots on net while exposing various cracks in the foundation known as the St. Louis Blues defense. Boston’s own blue line corps was also spectacular over the final 40 minutes, surrendering just a dozen shots during that time — six of those coming in the final four minutes as St. Louis tried frantically to net the equalizer.

There was also a seismic hit delivered by rearguard Torey Krug on the Blues’ Robert Thomas, a sequence that saw Krug have his helmet ripped off by David Perron in a defensive zone skirmish before he raced back up ice, dark brown hair flying behind him, before he absolutely pulverized Thomas with a clean hit just over the blue line. It sent the Garden crowd, already buzzing, into delirium.

“We weren’t perfect in the first ... and we weren’t expected to be,” said Clifton. “Eleven days off is a long time. But I thought we came out OK and got even better in the second, which is what you need.”

Down a pair and with his team needing a spark, Clifton took off toward the Blues net when a 3-on-2 break presented itself two minutes into the second period. Sean Kuraly spotted him doing so and the centermen flung the puck toward the net just as the affectionately known ‘Cliffy Hockey’ got there. The puck glanced off the boot part of his skate and in — St. Louis’ lead was cut in half just 76 seconds after Vladimir Tarasenko’s tally for the visitors.

“Sean made a great play driving wide, hit me back door and I stopped at the net. Luckily, it hit me and went in,” said Clifton.

A high sticking penalty by Joel Edmundson, the third taken by St. Louis, was killed off by the visitors, but not so when Oskar Sundqvist was caught cross checking Clifton at the 11-minute mark. After Grade-A opportunities to cash in by David Krejci and Patrice Bergeron just missed, McAvoy cashed in by rushing the cage and flicking a shot past Binnington at 12:41.

That two defensemen scored the first two Boston goals of the Cup Final — it was the second postseason tally for both — spoke volumes not only about Boston’s ability to get production from any player in the lineup, but their rearguards’ willingness to jump up into the play offensively when the situation dictated.

In both instances, it did.

Still riding that momentum in the third, Sean Kuraly scored what turned out to be the game-winner at the 5:21 mark before Brad Marchand sealed it with an empty netter.

That’s now eight straight playoff wins for the Boston Bruins, and the way they’re playing, the Blues are going to have a difficult time preventing them from imposing their will on them in this Stanley Cup Final.