In my nearly half century of Broadway theatergoing, I’ve never witnessed such a second-act reversal of fortune as what’s going on now at the Lunt-Fontanne Theatre, where “Charlie and the Chocolate Factory” opened Sunday.

Only in retrospective can you see glimmers of hope in the musical’s desultory first half. Just one song by Marc Shaiman and Scott Wittman hits the mark in this retelling of Roald Dahl’s tale of four children who win golden tickets to Willy Wonka’s long-shuttered chocolate factory.

In “What Could Possibly Go Wrong?” the media-obsessed Mike Teavee (the devilishly good Michael Wartella) and his doting mother, Mrs. Teavee (the divine Jackie Hoffman, fresh from her “Feud” triumph), present a witty send-up of a young Donald Trump Jr., as arrogant as he is obnoxious.

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In the first act, there’s also Mark Thompson’s stylishly spare set design, which completely eschews the tacky literalism of Disney Theatrical’s most recent stage ventures for children.

And then there’s Christian Borle in the very supporting role of the shop owner who keeps teasing the young Charlie (Ryan Foust) with offers of expensive chocolate, only to withdraw his beneficence at the last moment. Borle slyly delivers each of those mauvais mots, written by book writer David Greig, on a slippery silver platter.

Otherwise, the first act is interminable. While a game little performer, Foust (alternating at other performances with Jake Ryan Flynn and Ryan Sell) isn’t given much to do but be sweet in his unhealthy addiction to chocolate.

He’s also stuck with a generic mother (Emily Padgett) who panders with the song “If Your Father Were Here.” Yes, Charlie is a half orphan. Most tedious is the living arrangement of Charlie’s four grandparents (John Rubinstein, Kristy Cates, Madeleine Doherty, and Paul Slade Smith), all of whom sleep in the same bed. Yes, this unpleasantly cozy living arrangement is in the show’s source material, but visualized here its weirdness never intrigues — and the jokes about flatulence and bird poop don’t help.

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What’s great about Act 2 is that the mother and three of the grandparents completely disappear, and little Charlie takes a backseat to the far more colorful brats and their parents. And most important, Borle as Willy Wonka occupies center stage and never lets go, holding court as he blithely dispatches one kid after another to apparent death. In the new “Groundhog Day,” Andy Karl makes you forget about Bill Murray in the film version. Borle does Karl one better:  He makes us forget about both Johnny Depp and Gene Wilder.

Of course, it’s not all Borle. His Wonka material is good. So, too, are the staging of the multiple deaths by candy overload. “Veruca’s Nutcracker Sweet,” which shows the young Russian ballerina snob (Emma Pfaeffle) being torn apart by giant squirrels, is right up there with the doll scene from “The Tales of Hoffman,” as choreographed by Joshua Bergasse.

Wittman and Shaiman, along with veteran director Jack O’Brien, are the men who gave us “Hairspray,” after all. They understand the skewered malevolence of a Wonka. They and Greig don’t have a clue how to offer up the fuzzy goodwill of Charlie’s home life.

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Someday I’d like to know the negotiations that go on behind the scenes between songwriters and producers when it comes to sticking hit songs from the movie into the stage version. Do the creative people squawk and put up a good fight? Or do they just surrender and give the kids what they already know?

You can almost overlook the addition of Leslie Bricusse and Anthony Newley’s “I’ve Got a Golden Ticket” and “The Oompa Loompa Song.” They’re both minor stuff, and it just seems lazy not to have written new versions. To begin the show and then reprise “The Candy Man,” however, is all wrong. Images of Sammy Davis Jr. and the Rat Pack belong in another show about 3,000 miles away.