In the case of “Rocky,” let’s begin at the end. The electric final 15 minutes of the new musical based on Sylvester Stallone’s small-town Philly boxer, now open at the Winter Garden Theatre, are likely to inspire a heavy outpouring of adjectives: Game-changing. Jaw-dropping. Astounding. All are fair.

Preceding the high-voltage conclusion—a round-by-round battle between the idealistic Italian Stallion and world champ Apollo Creed that makes use of the theater space in a quite novel way—is an otherwise-workaday musical buoyed by enough built-in goodwill to lift it up the stairs of the Philadelphia Museum of Art, and beyond.

“Rocky” owes its success to three individuals. Foremost is star Andy Karl, late of “Jersey Boys” and “…Drood,” who packed on 12 pounds of muscle to embody the South Philly southpaw given a once-in-a-lifetime shot to go the distance against a heavyweight champ.

Karl’s optimistic fighter is a lovable goofball whose job as muscle for a local loan shark is compromised by his unwillingness to break thumbs. How awesome is Andy Karl? He sings and does chin-ups at the same time.

Director Alex Timbers (“Peter and the Starcatcher”)—who, lest we forget, was born two years after Stallone’s 1976 film won the Oscar for best picture—and choreographer Steven Hoggett (“Once”) have created fight scenes that are athletic and showy. Save for the non-regulation boxing gloves and other Broadway flash, Karl’s Balboa and Archie’s Creed look like they are duking it out at the Philadelphia Spectrum, jab by jab.

And actually, there’s a fourth fella without whom this would all be for naught: that’s Stallone himself, who shares a book writing credit here with Thomas Meehan, the author of “Annie.” (The two stories aren’t so different: one protagonist has a hard-knock life; the other spends her time hanging out with orphans.)

With his film, Stallone brought to life characters and created images that have stayed with us for four decades. Note the audible cheers at the Winter Garden when the second act begins with Rocky cracking eggs into a glass. We get the crooked fedora and the slabs of beef Rocky uses for training. We also get Rocky’s ramshackle apartment and his sheltered roommates: “Yo, turtles! Your old man’s home.” Timbers and his team craft an homage that, while not duplicating the film, hits all the touchstones.

As the curtain rises on the first act, a bout is in progress in a full-sized boxing ring in the basement of a South Philly Catholic church. In short order, we’re reacquainted with characters, some familiar (Margo Seibert is winning, if hardly mousy, in her Broadway debut as Adrian) and some not (Adrian has picked up a gaggle of pals at the pet store where she works, most notably confident Gloria, played by Jennifer Mudge).

Two characters beloved from the film don’t translate smoothly. The wonderful Danny Mastrogiorgio feels misused as Adrian’s drunkard and downright cruel brother Paulie. As Mickey, Rocky’s eventual manager, character actor Dakin Matthews comes off as too much of an opportunist, and that diminishes his likability. Archie’s Apollo Creed is a great showman, particularly when he struts down the aisle prior to the start of the championship match.

The music and lyrics by longtime collaborators Stephen Flaherty and Lynn Ahrens (“Ragtime”) are serviceable, if humdrum. “My Nose Ain’t Broken,” used to establish Rocky’s years training in obscurity, hardly seems to warrant a reprise, yet it gets one. The two best songs in “Rocky” are those not written for the musical: Bill Conti’s spirit-lifting theme, and “Eye of the Tiger,” used in a training montage that includes those iconic museum stairs. The latter was written for “Rocky III,” but don’t be confused—the Broadway musical otherwise tracks the plot of only the first film.

Some two hours in, Rocky and Apollo gear up to do battle, and there’s a substantive change of energy in the room. Broadcasters, camera persons and security guards buzz around. Trainers fret. Punches are thrown and blood is drawn. For whatever its flaws, “Rocky” the musical wins with a knockout final scene that is, guaranteed, unlike anything you’ve seen in the theater.

“Rocky,” with an open-ended run at the Winter Garden Theatre, 1634 Broadway. Tickets: $79-$143. Call 212-239-6200 or visit Telecharge.com.

Follow Robert Kahn on Twitter@RobertKahn