WASHINGTON — The perfect storm conspired to deprive the Mets of what should have been a sure victory Tuesday night.

Nolan McLean was handed a five-run lead after the Mets had batted twice, but somehow that wasn’t enough.

The rookie got knocked around, with a freak play interspersed, and soon the Mets defense turned leaky. Then the bats that had crushed it early just disappeared.

Poof, the lead vanished. The Mets lost 9-6 to the Nationals, snapping a three-game winning streak.

James Wood’s inside-the-park grand slam in the second inning sent McLean spiraling and he never recovered. In his worst career start he surrendered nine runs, six of which were earned, on eight hits and two walks with a hit batter over 5 ²/₃ innings.

McLean’s ERA jumped from 2.92 to 3.57, but by working into the sixth he kept the Mets from further overextending a taxed bullpen.

“I didn’t have my best stuff, but that is not an excuse for not going out there and competing better than I did,” McLean said. “They made some good swings on good pitches, I would like to execute a little better, though.”

Bo Bichette’s two homers in the first two innings had the Mets rolling with a 5-0 lead, but the good times soon ceased.

Foster Griffin recovered over the next three innings and the Nationals bullpen handled the rest.

Nick Morabito started in left field in his major league debut and finished 0-for-3.

Defensively he watched Wood’s shot hit his glove and then bound from the fence — as Tyrone Taylor converged with him — leading to the grand slam that shifted the game’s momentum.

“[Wood] hit it pretty hard and I went back and just missed it,” Morabito said. “It hit the palm of my hand, and I should have had it.”

A night earlier the Mets scored 10 runs in the 12th inning to rally for a 16-7 victory. There were plenty of noteworthy offensive performances.

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This night was quieter, with Bichette — who has underwhelmed during his nearly two months with the club — finally showing a spark before Juan Soto homered in the seventh.

Bichette’s first homer of the night gave the Mets a 2-0 lead in the first inning following Carson Benge’s leadoff single.

Marcus Semien brought in the inning’s third run with a sacrifice fly after Soto and Mark Vientos reached.

After Benge singled with two outs in the second, Bichette smashed his third homer in two games, extending the Mets lead to 5-0.

The blast was Bichette’s fifth this season and gave him the first multihomer game of his Mets career.

But most of the Mets lead disappeared with two outs in the second inning. McLean allowed a double to Jorbit Vivas and plunked Drew Millas on an 0-2 pitch.

After Nasim Nuñez singled to load the bases, Wood cleared them (the second inside-the-park grand slam in Nationals history).

“The 0-2 hit-by-pitch was kind of like the killer there,” manager Carlos Mendoza said. “It ended up costing a lot in that inning.”

McLean’s troubles were just starting. José Tena homered leading off the bottom of the third to tie it 5-5 before CJ Abrams walked and Daylen Lile singled.

Luis Torrens’ passed ball gave the Nationals the lead and Vivas’ sacrifice fly put the Mets in a 7-5 hole.

Sloppy defense by the Mets in the fourth widened the gulf.

Semien booted Tena’s grounder and then unleashed an errant throw to second, putting runners on second and third. Abrams followed with a grounder to Bichette, who fired home.

The ball hit off Torrens’ glove for an error on the catcher. Two runs scored on the play, extending the Nationals lead to 9-5.

“Not good,” Mendoza said of the team’s defensive play. “The past four days or so we are making errors on routine plays. We are a good team defensively, we have shown that, but we are going through a stretch right now where you have got to clean it up.”