Sometimes there's a show, or a movie, that is so original in its form and passionate in its execution that one forgives its fundamental flaws. Such is the case with If/Then, the direct-from-Broadway musical by Tom Kitt, with book and lyrics by Brian Yorkey, that just opened last night at the Orpheum. It is a complicated and inspired piece of musical theater, and a perfect vehicle for a strong female singer just hitting middle age, like Idina Menzel, who can carry a show in which she alone sings a half dozen belt-to-the-rafters showstoppers.

Intact from the Broadway run in this first national tour is the core cast of four, including Tony Award winner Menzel — now most famous as the voice behind "Let It Go" from Frozen, and also Tony and Drama Desk nominated for this role — and fellow original Rent cast member and Broadway star Anthony Rapp, both playing roles that feel tailor-written for them.

Menzel plays Elizabeth, who arrives in New York City after a divorce and nine years living in Phoenix, at age 38, looking to start over. The conceit of the show is that based on decisions she makes on her first day in town, and the choice between the influences of two friends — one of whom puts love before all else, and one of whom puts work first — her life could take two very different paths, as could, consequently, the lives of those same friends. To accomplish this dual trajectory on stage, she adopts a different nickname for each timeline, Liz and Beth. Liz wears glasses and falls almost immediately in love with an army doctor she meets in the park one day, and Beth does not wear glasses, remains single, and takes a job at the city's planning department.

Joining Menzel and Rapp from Broadway is the confident and hilarious LaChanze as Kate, Elizabeth's new best friend and neighbor who pushes her toward love, and James Snyder as the soldier Liz falls in love with and marries. Snyder's high tenor voice pairs especially well with Menzel's booming alto, and the two were, not surprisingly, chosen as Best Onstage Pair in a audience poll.

The parts of the show that work brilliantly are many. Kitt and Yorkey's songs are as melodically complex and breathlessly dense as those they wrote for Next to Normal, only here instead of describing one woman's descent into mental illness they express the chaos and heart-wrenching push and pull of loves that are found, worried over, drunk in, misjudged, and lost. Several of the numbers — including the opening/closing bookend theme "What If?", Snyder's first solo number "You Never Know," "Love While You Can," and Menzel's final barn burner of a ballad "Always Starting Over" — feel like they will enter the Broadway canon for a long time to come. And I can't neglect to mention the finest — and only — show tune ever written about urban planning.

The set by Mark Wendlend, who also designed the spare modern frame of a house for Next to Normal, is elegant and versatile, with modular frames made of what look like steel I-beams that evoke urban planning, urban spaces, the subway, and cramped New York apartments — as well as a catwalk that raises and lowers over the stage to evoke bridges and elevated roadways.

The cast's performances, including those by the stellar ensemble, rarely feel forced. And Menzel goes through a dizzying rollercoaster of believable emotions throughout the show's two-plus hours that make it all the more impressive that she can still sing by the end.

So when it comes to fundamental flaws, I can only really point to the structure of the play itself and the way the dual-timeline thing is executed, both in the writing and in the direction by Michael Greif. Because the production aims for such seamless transitions between the Liz and Beth plot lines, and because the audience has to constantly listen for the nicknames and pay attention to whether Menzel is wearing her Liz glasses or not, it makes for some seemingly unavoidable confusion from the moment Liz and Beth's trajectories part ways in Act One. She is, after all, the same character, singing similar songs of longing, and her two fates are playing out simultaneously — a narrative leap that is asking a lot of all but the most attentive audience member. There's even one song in Act One sung by both Liz and Beth, and while the set spins and transforms the scene very effectively, it's a little harder to grasp how we're leaping between the character's two parallel lives mid-song. Not to mention the fact that Rapp's character, a housing activist with a crush on Liz/Beth since college, is bisexual, and in one storyline he and Beth become romantically entangled, while in the other he meets a boyfriend whom he ultimately moves in with, only adding to the confusion. As Liz says early in the show, regarding how she doesn't believe in bisexuality, "Pick a side!"

Again, the complexities built into this premise are as stimulating as they are occasionally baffling, but they likely mean that the show — which ran for a solid but likely not profitable 400 performances in New York — may end up getting relegated to the archive of misunderstood Broadway curiosities until such time as someone dusts it off, in a decade or two, and finds another singer of Menzel's caliber to take it on, with a fresh vision for how to delineate the characters' double tale.

The play reminds me in that way of Sondheim's brilliant, and initially clumsily executed musical Merrily We Roll Along, which tells the story of a threeway friendship, backwards, from when three friends are middle aged, semi-estranged, and bitter, to 20 years prior when they were young, inspired, and just starting out in the world together. Producers in the early 1980s originally cast young people in the roles, and ultimately had them wearing t-shirts with their character names on them to keep audiences from getting confused. The problem was solved, much later, in a critically acclaimed London revival in 2013, with a minimally revised script and the casting of older actors who embodied the older selves of the beginning of the show much more believably.

I could see something similar happening with this show — and the music really is too good to be shelved for too long.

If/Then plays at the Orpheum through December 6. Get tickets here, or check out the TodayTix app to find rush seating at a discount.