When Idina Menzel first appeared onstage, her presence abruptly bringing the swelling overture to a stark silence, there was perhaps no greater applause during the nearly three-hour performance of “If/Then” than when she delivered the show’s opening line: “Hey, it’s me.”

And that immediate enthusiasm is completely warranted, because a national tour of a star-studded, Tony Award-nominated musical (currently at the Orpheum Theater in San Francisco) is rarely filled by its original cast members. Menzel and Anthony Rapp, reunited again after co-starring in “Rent,” are the undisputed stars of the show — stars who perhaps outshine the show itself.

Created by the duo behind the Pulitzer Prize- and Tony-winning rock musical “Next to Normal,” Tom Kitt and Brian Yorkey’s show revolves around Menzel’s character, Elizabeth, a city planner in her late 30s who leaves her husband in Phoenix to start over her life in New York City. The show opens on Elizabeth’s first day in New York, when she is greeted by both her college-sweetheart-turned-best-friend, Lucas (Anthony Rapp), and her new across-the-hall neighbor, Kate (LaChanze).

From there, Elizabeth is given two choices: attending an anti-gentrification protest with Lucas or going to a concert in Brooklyn with Kate. Her choice dictates the course of her future — whether she gets a job, meets the man of her dreams or starts a family. By the end of the show’s opening number, Elizabeth finds herself ruminating on all the possibilities, asking herself, “What if?”

The audience, however, does not have to wonder “what if,” as the musical explores both of Elizabeth’s possible paths. If she follows Kate, she becomes known as Liz and brushes shoulders with a potential suitor. But if she follows Lucas, she becomes known as Beth and receives her dream job as a city planner, working under her former graduate school boyfriend.

As Kitt and Yorkey have a propensity for writing music suited for a demanding female vocalist (see Alice Ripley as Diana Goodman in “Next to Normal”), Menzel’s execution was unsurprisingly effortless. Menzel’s sheer presence seemed to dwarf that of the ensemble, and simplistic songs and melodies proved not to challenge her incredible vocal ability.

Additionally, much of the music was almost audibly indistinguishable. Many of the songs sounded extremely similar and seemed to focus more on narrating the fast-paced storyline than on the emotional responses of its characters. It is as if Kitt and Yorkey were attempting to recreate the clever storytelling quality of “Next to Normal’s” soundtrack but lacked the emotional depth to execute it the same way.

The ensemble musical performances seemed average and, at times, a bit underwhelming, but the show truly shined through its stars. LaChanze stands out as a pointed and pragmatic kindergarten teacher in “No More Wasted Time,” her soulful voice echoing the poignant message of grabbing life as it comes and never letting go. Rapp is earnest and honest in his performance of “You Don’t Need to Love Me,” his tender tenor voice caressing each line with such sincerity that you can’t help but feel a pang of pain in your chest when he sings, “You don’t need to love me the way that I love you.”

But of course, it is Menzel’s emotionally driven rendition of the musical’s penultimate song, “Always Starting Over,” that serves as the show’s most memorable moment. “Always Starting Over” seemed to be the only song that was fitting of Menzel’s unfathomable vocal range. As she sang, the stage was bare, dressed only by the highs and lows of her soaring, unmovable voice. She belted lyrics that seemed to sum up the show’s central message: “If we’re always starting over every brand new morning, then we’re always starting out with the end in doubt.” And as Menzel crescendoed into the song’s final, climactic note, it was difficult not to draw comparisons to her iconic role as Elphaba from “Wicked” soaring above the stage during the final moments of “Defying Gravity.”

Despite its minor flaws and somewhat forgettable tribulations, “If/Then” is still worth seeing, purely thanks to the undeniable collective talent of its central cast. To see Rapp and Menzel reunite onstage is the stuff of a theater lover’s wildest dreams, and for them to be part of the national touring cast truly makes this production a treasure.

“If/Then” is playing at the Orpheum Theater through Dec. 6.