WASHINGTON — The Dark Knight had been knocked to the ground, his lance broken, his horse having long ago bolted through the center-field gates.

Then his Mets teammate Kirk Nieuwenhuis, picked back off the baseball refuse heap, a fifth — or is it sixth? — outfielder, came to the plate as a pinch-hitter. He had hit just three home runs this season, and all had come in one game. The rest of his season had not been magical, either.

But he took a low, hard swing and launched a ball on an arc into the right-field stands.

That improbable shot put the Mets up by a run, the winning run as it would happen. It was a thunderbolt stroke in an 8-7 win over the Nationals on Tuesday night, capping a comeback from a 7-1 deficit. Minutes after the game, loud cheers and chanting could be heard from within the Mets’ locker room, Nieuwenhuis at the center of it.

Not so terribly long ago, Nieuwenhuis lit the minor leagues afire, a 6-foot-3 tight-end-size outfielder who never stopped running or swinging. Terry Collins worked in the Mets’ development system back then.