What a night for Pete Alonso and the majority of the 28,097 Mets fans on hand.

What a gut punch — or maybe a punch directed a bit lower — for the Mets.

In yet another surreal scene during a series filled with them, Alonso crushed a milestone, game-tying home run at Citi Field — and bathed in the adoration of Mets fans who had just watched him help steal a game from the team they root for.

Alonso crushed No. 300 to tie it in the ninth.

It was another home run — an inside-the-parker from Coby Mayo in the 11th inning — that just about sealed it for the Orioles.

And after all that, the Mets might have felt as bruised as A.J. Ewing, who smacked into the wall and was slow getting up after trying to catch the Mayo drive that led to the 7-5 Mets loss Tuesday in Queens, where the fans showed dramatic swings in allegiance.

The Mets are already mathematically eliminated, but Tuesday sure did not feel meaningless.

With one out in the top of the ninth of a game the Mets led by one, Alonso got a 2-0 ghost forkball from Kodai Senga that did not disappear and instead was quite visible in the middle of the strike zone.

Alonso turned on it, demolishing a high-arcing blast to left for his 300th career home run in a ballpark that has seen 124 of them, but only one in which Alonso provided a run for the opponents.

The swing was less jarring than the reaction: Alonso hurled his bat toward the Orioles dugout and circled the bases to mostly cheers.

“Pete Alonso” cheers broke out until Alonso — in a visiting ballpark, having just tied the game 3-3 — took a curtain call from a fanbase that does not have much to root for during September of a lost campaign in which they already have been eliminated, and thus decided to root for the one who got away.

As Alonso disappeared back into the dugout, those cheers transitioned into a loud, piercing jab toward the person who helped shoo the Polar Bear away: a stadium-wide round of “Fire Stearns.”

After both teams traded runs in the 10th, the Orioles — who are still alive and fighting for a wild-card spot — ran away with the contest against Jonathan Pintaro. The largest blow was Mayo’s drive chasing Ewing to, and almost through, the center field wall, smacking hard against it and then hitting the warning track dirt. Ewing appeared to be OK, but by the time he and the Mets outfield recovered, Mayo had rounded the bases for a three-run inside-the-park home run.

For Mets fans hoping for another Alonso moment in Queens and mere drama in a season that no longer has much, the ninth-inning swing offered plenty.

For the Mets — a group that has been the undercard in its own ballpark, with Jonah Tong hearing boos a day earlier when he walked Alonso rather than serving up a home run — it is hard to imagine the season winding down any worse.