The crowd stood and rocked Citi Field, the theme to “The Natural” mixing in to create a soundtrack of hope, appreciation, euphoria.

Pete Alonso had turned the first pitch of the bottom of the fourth — a 92 mph Yu Darvish fastball — into history. The Polar Bear outdoing a Cub. Alonso went 402 feet to the opposite field, cleared the right-field wall and Todd Hundley and Carlos Beltran. His 42nd homer the most ever in any of the Mets’ 58 seasons.

The Mets led an important late August game 1-0 and then …

Nothing.

By the eighth inning, the soundtrack was boos. By the ninth inning, more than half the 34,158 who had come with such playoff-race excitement were gone, so golf applause greeted J.D. Davis’ relatively meaningless two-out homer. The Cubs won 5-2 and the Mets have to hope there was not a Met-aphor for a whole season within these three hours — joy about Alonso, frustration about pretty much everything else.

Marcus Stroman is now 5-for-5 in failing to register even a quality start as a Met. He yielded a pair of two-run homers in six innings (the first to Addison Russell in the fifth giving back the lead Alonso created about 10 minutes earlier). He said of his Mets work to date: “I need to be better.” The Mets actually had won Stroman’s first four starts because the offense generally covered for him. But the lineup has gone missing at the worst time. It also needs to do better — way better.

The Mets have managed just 11 runs in the past five games (five in one game against the Braves) and have now lost four straight to fall three behind the Cubs for the second wild card. Kyle Hendricks, who has permitted one run in 19 ¹/₃ career innings against the Mets, faces Noah Syndergaard Wednesday before Jon Lester-Jacob deGrom Thursday.

“This is an opportunity for us,” Mickey Callaway said. “We have to come out and win the next two.”

Indeed, there is opportunity. For all the talk about the bunched wild-card race, the reality is the Diamondbacks and Giants have fallen out of contention. The Phillies and Brewers — their pitching spent, broken or underperforming — look like tired marathoners at the 20th mile.

The Nationals are comfortably entrenched as the first wild card, leaving the Cardinals, Cubs and Mets for two spots — the NL Central title (that the Mets obviously can’t win) and the second wild card.

But to get to October the Met offense needs a quick course correction. Alonso’s homers to tie and break the franchise record have come in losses. The team is hitting .202 in the last five games. The Mets are 10-for-64 (.156) with men on base in that span, including 0-for-9 in this pivotal opener against the Cubs. They are 5-for-35 (.143) with runners in scoring position, including 0-for-5 against Chicago.

“We are not getting the timely hit,” Callaway understated.

Aside from Wilson Ramos, who extended his hitting streak to 20 games, and Juan Lagares, who did not start Tuesday, the Mets lineup has gone mainly silent.

Darvish, who had not gotten an out past the seventh inning since July 21, 2017 — 10 days before he was traded from the Rangers to the Dodgers — used his vast catalogue of pitches to hold the Mets to one run (the Alonso homer) in eight innings. The Mets’ lone hit in the final three innings was Davis’ futile homer.

The Mets have the kind of starting pitching to win and their bullpen has performed much better — in part because the starters have logged innings to protect the relievers and allow them to be used in more favorable situations. So this cannot be an extended period in which the offense vanishes.

There could be reinforcements nearing. Brandon Nimmo might be back by the weekend. Robinson Cano took batting practice on the field Tuesday. Jed Lowrie has begun to play the field in minor league rehab games.

But this is about the group that is here, that has the Cubs in front of it for the next two games and, thus, a chance to put the Mets anywhere from one game back of that second wild card to five back.

The Alonso homer was a another feel-good moment for a rookie who has brought a lot of that to the Met fan base in 2019. But if this season is going to be remembered for more than that (and frustration), the offense must awaken.

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