NEW YORK – Pete Alonso’s improbable home run set the tone for the evening.

The Mets first baseman hit a first-inning slider from the Dodgers’ Jack Flaherty at his feet in deep center field, a three-run, 400-foot explosion that accelerated the Mets’ offense into an eventual 12- 6, season-saving win in Game 5 of the National League Championship Series on Friday.

The home run wasn’t so much a pitching error as it was a hitting miracle.

Alonso made contact on a pitch just 12 feet above the ground, the second-lowest ball hit for a home run in the postseason in the pitch-tracking era (since 2008).

“I was just looking for something in the middle of the plate,” Alonso said when asked to explain how he got the barrel of the bat on Flaherty’s pitch. “I didn’t really realize how low the field was. I got caught a little bit in front. … Honestly, it’s inexplicable. It’s the magic of the postseason. And I’m just glad I got it right .”

The home run capped off an incredible night for the Mets offense, with not a single New York batter striking out. It was the first time a team had not struck out in a postseason game since the Los Angeles Angels played the San Francisco Giants in the 2002 World Series.

“We didn’t go crazy?” said outfielder Jesse Winker when informed of the achievement. “We didn’t get a strikeout? Wow. I don’t think I’ve been a part of a game like that. That’s awesome. We’ve just got to keep doing that.”

The turnaround against Flaherty was striking. He shut out the Mets over seven innings in a Game 1 win, but lasted just three innings in Game 5, giving up eight runs, eight hits and four walks.

“We didn’t chase his secondary pitches,” Mets manager Carlos Mendoza said. “We know he’s got that slider and that knuckle curve, and he’s going to try to make us chase it, and we didn’t do that today. And when he got in the zone with his fastball, we were ready, and that’s the key.”

After the game, Dodgers manager Dave Roberts revealed that Flaherty was not feeling his best.

“He obviously wasn’t on his toes,” Roberts said. ‘He’s been fighting something. He’s been a little confused. So I don’t know if that’s in the stuff, the speed.’

Flaherty averaged 90.4 mph with his fastball in Game 5, up from 92.6 in Game 1. His average in the regular season was 90.3 mph.

After the defeat, the veteran right-hander did not elaborate on how he felt during the match, but offered the following explanation for his troubles: “For the first time in a while, I felt like I was letting the game get to me a little bit and did not make the adjustments in the game.”

Roberts could have pulled him earlier, but knowing his team had a cushion in the series, he left him in as the Mets built a big lead.

As much as Flaherty struggled, credit goes to the Mets, who had 14 hits, including four from Starling Marte and three from Francisco Alvarez.

“When it comes to hitting, at that point you just have to trust that you’re going to take advantage of the mistakes the pitcher makes,” Marte said. “And fortunately that’s what I was able to do in terms of getting the results.”

Although the Dodgers didn’t go quietly, the 43,841 fans at Citi Field saw the Mets respond every time.

Dodgers No. 9 hitter Andy Pages hit two home runs and Mookie Betts’ explosion in the sixth inning cut the deficit to 10-6, but Jeff McNeil’s sacrifice fly in the bottom of the sixth and Marte’s RBI single in the eighth provided New York with its margin of victory.

“It was just an incredible execution,” Alonso said. “That’s what it comes down to. I thought the most impressive thing was that every time they scored, we had an answer for it.”

That performance was something shortstop Francisco Lindor implored his team after the loss in Game 4, when the Mets were down 12 and went 0-for-10 with runners in scoring position. On Friday, New York went 5-for-19 with runners in scoring position.

Lindor was asked what he liked most about the difference between the two matches.

“The quality of at-bats we had,” he said. “The intensity with which we came. We understood that it was a do-or-die game and that we had to give everything we had. And we did that.”

That urgency was also reflected in the way Mendoza used his bullpen, as Ryne Stanek went 2⅓ innings, his longest stint of the season, and Edwin Diaz pitched the final two.

“You know you have to go (longer), so try to hold yourself back a little bit and try to stay as even as possible and let him drive,” Stanek said of the longer stint. “I enjoyed it.”

The win sends the series back to Los Angeles on Sunday for Game 6. On paper, the Mets have the starting pitching advantage, as Sean Manaea gets the ball on five days’ rest; the Dodgers are planning a bullpen game.

“We’ve had our backs against the wall all season,” Mets designated hitter JD Martinez said. “Why would this be any different?”