Something electric happens at the end of “Rocky” that gets theatergoers on their feet and writers scuttling for exclamation points:

A boxing ring descends from the rafters, then glides into the orchestra!

There’s hooks, punches and blood — and a Jumbotron!

And then: “Adriaaaaaaaan!”

Director Alex Timbers (“Bloody Bloody Andrew Jackson”) earns his keep right there. If you could win a Tony based on just 20 minutes, “Rocky” would be a shoo-in.

Problem is, that finale is preceded by an hour and a half of less thrilling moments.

Turning Sylvester Stallone’s beloved tale into a musical wasn’t the most obvious idea, but some producers believed in it — even if they were from Germany, where the production opened in November 2012.

Thursday night, the musical made its Broadway debut with a cast headed by Andy Karl as underdog Rocky Balboa and Terence Archie as heavyweight champ Apollo Creed. And it turns out that a boxing musical wasn’t such a crazy notion, after all.

Plot-wise, book writers Stallone and Thomas Meehan (“The Producers,” “Hairspray”) hit all the film’s classic scenes: Rocky punching slabs of beef in the Shamrock Meats freezer, running up the steps of the Philadelphia Museum of Art, and gulping down raw eggs for breakfast.

“Eye of the Tiger” pops up as well, even if it’s actually from “Rocky III.” Too bad using that song and “Gonna Fly Now” (a k a Bill Conti’s original theme) only underline how Stephen Flaherty and Lynn Ahrens’ score lacks energy, not to mention soul.

That absence is glaring for a story set in 1975 Philadelphia. We get only a whiff of it in Apollo’s “Patriotic,” in which he boogies with a scantily clad female entourage, but Ahrens and Flaherty don’t have a funky bone in their bodies.

Fortunately, they’ve written some sweet ballads, including a couple of lovely duets for Rocky and Adrian (“The Flip Side” and “Happiness”). This is important, because “Rocky” really is a tender love story camouflaged under a big, burly exterior.

As the Italian Stallion, Karl follows the Stallone model — a big galoot with a heart of gold — but he makes it his own. He has a cool-cat ease and a warm, evocative singing voice. We believe in this irrepressible Rocky and his glass-half-full philosophy: “I got a crooked employer/And a job I hate,” he sings. “But hey/My nose ain’t broken.”

He’s well matched with Archie’s cocky Apollo and Margo Seibert’s Adrian, even though she isn’t as pathetically lonely as the movie’s semi-recluse.

Here she has a friendly boss at the pet store she works in — Gloria (Jennifer Mudge), who’s dating Adrian’s brother, Paulie (Danny Mastrogiorgio).

In what feels like a cop-out, Paulie’s been softened from major creep to minor irritant, and doesn’t even get a song — whereas coach Mickey (Dakin Matthews) scores a nostalgic, momentum-sapping number, “In the Ring,” about boxing’s heyday.

Quibbles, quibbles: The epic brawl wipes them all out, and resets the audience’s memory so we leave on a Himalayan high. “Rocky” does win, after all.