Idina Menzel as Elizabeth and the company. Photo by Joan Marcus.

Idina Menzel as Elizabeth and the company. Photo by Joan Marcus.

Boy, I really loved If/Then when it was the 1998 movie Sliding Doors.

In that film, from which the new Idina Menzel vehicle borrows heavily, a pre-macrobiotic Gwyneth Paltrow lives two parallel lives in London, depending on whether she caught her train home on time or not. This small change in outcomes basically makes all the difference in whether she catches her boyfriend cheating on her, starts a new career, dyes her hair blonde, etc.

It was a cute, smart romantic comedy that managed to avoid killing itself trying to extract “meaning” out of which path was the best. Indeed, the sense that all things inevitably work themselves out in a way that is very reliant on fate—that some paths are simply destined to cross—was one of its most appealing aspects.

Playing through December 8 at National Theatre, If/Then is so similar to Sliding Doors, it makes you wonder why they didn’t just call it Sliding Doors: The Musical. (I guess they figured it was fine to steal from a movie that only grossed around $9 million.) But it enjoys none of its predecessor’s light touch.

Menzel plays Elizabeth, an urban planner who’s just steps away from Rain Man in her inability to pick up on social cues combined with a predilection toward unnecessary math problems. “How do I quantify coffee?” she says, straight-faced. Of course, everyone seems to love Elizabeth and talk about how great she is at her job, though we never see any tangible evidence as to why.

Elisabeth’s destiny is not determined by the sliding doors of New York’s subway, of course, but by whether she goes by the nickname “Beth” or “Liz.” In one identity, she gets married to a doctor who reminded me of a less rapey, modern-day Greg, Joan’s husband in “Mad Men”; in the other, she remains commitment-phobic but wins an urban planner award (we can’t have it all, ladies). Of course, all of these factors would be fine if the music itself was catchy or even memorable. But after more than two hours, you leave the theater completely unable to hum back a single number.

Thematically, If/Then at times feels like privileged, watered down version of Rent, and not just because it also happened to also star Menzel and Anthony Rapp (who seems stuck forever in the same role). While Rent is remembered most for its portrayal of protagonists living with HIV/AIDS, and rightly so, a good portion of that landmark piece was also devoted to the complicated politics of gentrification — and City Hall’s war against the poor. But rather than speaking truth to power, like Rent did in its own daring (for the time) way back in the mid-90s, If/Then bizarrely condescends more often than not, especially toward anyone who doesn’t reside in the five boroughs (or, let’s be honest, in posh-Brooklyn or Manhattan).

“I’m actually kind of normal (in Nebraska),” says a soldier who tries to get Menzel’s number for way longer than would any single man in a city with the kind of male-to-female singles ratio of New York City. Later, someone says, “I didn’t even know there was a river in Nebraska!” I mean, gosh. I didn’t even know there were sidewalks in New York that don’t smell like urine and public masturbation!

I love New York and have been to that city often, but we’re barraged over and over with “insider-y” jokes that fall flat. Like: “Meeting a man on the subway is like doing your Christmas shopping at the Port Authority.” Huh?

Also, references to things like the dividing line between Bushwick and Queens, I suspect, will already have no meaning whatsoever by the time this premieres in March, given all the gentrification and development rapidly taking place there.

As for the music and lyrics, If/Then clearly owes a thanks to the earliest musical about upper middle class relationship angst and alienation in New York — Stephen Sondheim’s Company, which features still-potent songs like “Another Hundred People,” “Sorry Grateful,” “Marry Me A Little” and “Being Alive,” the latter of which makes this particular writer cry every time. It’s probably unfair to hold anyone up to Stephen Sondheim’s standard, of course, but then this is the same writing team behind Next to Normal, which won the 2010 Pulitzer Prize in Drama, so it’s not as if we’re talking about hacks.

And while I’ve already probably made my point, I took a lot of angry notes, so I should also mention that the dancing is forgettable, too, with choreography mostly consisting of the cast’s talented ensemble being forced to walk across the stage like extras, pretending to sip coffee and do other yuppie-type things.

At any rate, many in the audience on the night I attended seemed to be there solely for Menzel (whose opening line, “Hi, it’s me,” got so much applause she had to pause her dialogue). These people will probably still enjoy the show (and the pricey ticket) regardless of what anyone says about it online or in print.

To her credit, Menzel, a true pro, delivers a show-stopping vocal exhibition toward the end that, a la Wicked’s “Defying Gravity,” sort of made time, and my own heartbeat, standstill. That powerhouse voice emanating from her always-smaller-than-conceivable frame makes the whole room shake.

Too bad it’s wasted on such a mundane production.