The “Gotterdammerung” of children’s theater has finally made it to Broadway. If you’re an adult without kids and have somehow been able to avoid the “Harry Potter” phenomenon in novels, movies and theme parks, the good news is you don’t have to see “Harry Potter and the Cursed Child, Parts One and Two,” which opened Sunday at the Lyric Theatre. Without prior knowledge of the “Potter” story, you won’t be able to understand what’s happening on stage anyway.

A top Broadway designer sat behind me in the Lyric Theatre, and before Part Two began, he asked to anyone in earshot, “Can somebody tell me what the hell is going on?” A Potter parent in the audience attempted to explain.

Just so we get the credits straight, “Harry Potter and the Cursed Child” is based on an original new story by J.K. Rowling, Jack Thorne and John Tiffany. The six-hour, two part was written by Thorne and Tiffany directs.

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Rowling, Thorne and Tiffany have taken the very sophisticated approach of plunging you right into the maelstrom of the wizard phenomenon. Forget about Harry Potter 101. This stage show is for advanced students who know every character, previous plot twist and the difference between Death Eaters, Muggles, Aurors and Dementors, among many other magical creatures.

Despite the show’s reported record-breaking $68 million price tag, Tiffany has wisely chosen to give the “Cursed Child” a low-tech look. As long as parents don’t have to take out a mortgage to purchase the tickets, the two-part play is great children’s theater that in no way attempts to replicate the movies.

The stage versions of “Mary Poppins” and “Chitty Chitty Bang Bang” dumbed the imagination with their literal interpretations. Tiffany avoids that visual trap by suggesting a train, for example, rather than presenting one on stage. The Hogwarts School of Witchcraft and Wizardry is little more than two moving staircases, and yet, in one illuminating sequence those steps conjure up dozens of locales in the imagination of any theatergoer, young or old.

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Photo: Manuel Harlan

Just as Dorothy in “The Wizard of Oz” just wants to go home to Kansas, “Cursed Child” wears a troubled heart on its black robes. In both wizard classics, the human emotions, not the special effects, propel the story forward.

Harry Potter (Jamie Parker), now middle-aged, experiences the parental pangs of watching his adolescent son Albus (Sam Clemmett) go off to Hogwarts, his alma mater. Harry is now a bona fide rock star of wizardry, so how can Albus ever measure up? Why, he can’t even raise a broom without using his hands! “Cursed Child” is all about how an ordinary son tries to measure up to a celebrity parent. Along the way, Albus befriends Scorpius Malfoy (Anthony Boyle), the son of Harry’s school nemesis, Draco Malfoy (Alex Price), and nearly brings back the rule of the dreaded Lord Voldemort (Byron Jennings) in the process.

Thorne sometimes goes overboard with the angst-ridden father-son psychobabble:  “I’m operating without wires here,” Harry tells Albus. “Most people at least have a dad to base themselves on — and either try to be or try not to be. I’ve got nothing — or very little. So I’m learning, okay? And I’m going to try with everything I’ve got — to be a good dad for you.”

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Photo: Manuel Harlan

It doesn’t take many speeches like that to bog down an already long show. As if to satisfy “Potter” fans, Thorne also takes up time introducing characters important to the novels that don’t serve the plot of “Cursed Child.” Albus Potter’s siblings, for example, figure prominently in the first 30 minutes of the play, only to disappear.

Albus’ need to prove himself involves some disastrous time travel. Here is the one place that Christine Jones’ monumental set design disappoints. “Cursed Child” imagines an alternate world, the result of events being altered by Albus and Scorpius’ travel back through time. “It’s a Wonderful Life” and the “Back to the Future” movies do something similar, but stunningly visualize it. That darker alternate world in “Cursed Child” is indicated through a few fascist design elements and minor changes in costumes (by Katrina Lindsay) and wigs (by Carole Hancock). It’s not enough.

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Photo: Manuel Harlan

Two years ago, Parker and Boyle won Olivier Awards for their acting in the original London version of “Cursed Child,” and were joined in that winners’ circle by Noma Dumezweni, who reprises on Broadway the role of the pompous but brilliant Hogwarts alum Hermoine Granger.

These three actors and others in the production give fine performances, but in a Broadway season replete with great acting it’s difficult to see any of them being singled out come Tony time.

What’s definitely prize-worthy is the fantastic redesign of the Lyric, a modern barn if ever there was one on Broadway. The theater now feels much more intimate, with the balconies and boxes hugging a reduced orchestra section. The décor incorporates “Harry Potter” design elements, but they’re unobtrusive and would accommodate a revival of any new or vintage musical once “Cursed Child” leaves the Lyric a few decades from now.