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Sandwiched in between an electric effort from rookie Nolan McLean on Wednesday and the much-anticipated debut of Jonah Tong on Friday was pure slop.

After one of their best series of the season, the Mets responded with two of the worst half-innings of their season.

There were several dropped balls, slow tags, botched plays and three charged errors that led to five unearned runs.

It all added up to one disappointing and sloppy loss for the Mets in a 7-4 setback to the Marlins in front of 37,975 annoyed fans at Citi Field on Thursday.

The Mets (72-62) could not maintain the momentum from McLean’s masterpiece that capped a sweep of the Phillies, who moved back to five games ahead in the NL East.

A careless, three-run top of the third inning dug the Mets a hole that they climbed out of when Pete Alonso clubbed his 30th homer of the season, a two-run shot that tied the contest in the fifth.

But on this night, not even a Mets lineup that has been rolling could outhit its defense’s issues.

The game went sideways in a seventh inning whose tape the Mets will not submit to the Hall of Fame.

The frame’s misplays began with Alonso, who fielded a ground ball with a runner on first base and spun to begin a double play.

But in the process the ball popped out of his glove, and instead of two outs the Mets recorded none.

Agustín Ramírez then hit a bullet off Gregory Soto to left field that appeared to be struck too hard for Jakob Marsee to score from second — except Brandon Nimmo booted the ball and had to bend twice to scoop it up, letting the go-ahead run score.

Next to drop the ball was catcher Hayden Senger, who tried to backhand a Soto sinker and watched the ball carom off his mitt for a passed ball that moved both runners up one base.

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With one out and runners on second and third, Otto Lopez shattered his bat on a ground ball that sent a shaded-in Jeff McNeil to his left.

The second baseman fielded and threw home for a second time on the night and for a second time fired too late, a slow tag from Senger ensuring another run scored without an out recorded.

After a sacrifice fly, three unearned Marlins runs had scored that proved the game’s difference.

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The seventh inning was the last painful one for the Mets’ defense, but it was not the first.

Their problems in the third inning hurt Clay Holmes, who also hurt himself.

On this night, the righty was OK as a pitcher and worse as a catcher.

In that third, Holmes plunked Xavier Edwards, who easily stole second and went to third on a groundout.

With one out, the Mets moved in their infield as Hicks sent a chopper to McNeil.

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The second baseman charged and might have had a chance at Edwards at the plate with a perfect throw — but his attempt was imperfect, on the first base side, as a run scored and Hicks reached safely.

After a groundout moved Hicks to second, Troy Johnston hit a chopper to Alonso that should have ended the inning.

But Alonso retreated, fielded and threw overhand to Holmes, who may have been anticipating an underhand toss.

Holmes dropped the would-be out, and the ball ricocheted into foul territory to allow the go-ahead run to score and Johnston to reach second.

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Lopez then smoked an RBI single into left to plate another unearned run.

Holmes, who had allowed one soft run (on two infield singles and a sacrifice fly) in the first, allowed four runs (two earned) in five innings in what might be his final time pitching on four days’ rest.

A sixth starter in Tong is set to join the rotation Friday.

A potential Mets rally in the eighth was short-circuited when center fielder Derek Hill made a remarkable diving play to retire Cedric Mullins — the type of effort that eluded the Mets defense all evening.