The sublime talents of soprano Idina Menzel are showcased to the hilt in "If/Then," an impressive new tuner by Tom Kitt and Brian Yorkey that gives new meaning to the term "book-heavy musical." Bowing in a pre-Broadway tryout at D.C.'s National Theater, it tells a story so convoluted that the plot lines are color-coded. The plodding elements of the complicated yarn are among several fixable concerns that would advisably be addressed en route to Gotham.

To be fair, a cerebral examination of life's possibilities, and the consequences of chance occurrences, is the challenging undertaking from the talented "Next to Normal" team. The nuances of probability are among the weighty topics explored.

Menzel plays a 38-year-old Arizonian named Elizabeth who has relocated to New York City, where fateful choices seemingly abound. Author Yorkey has devised two alternative paths for her to pursue, and unveils them simultaneously. As "Beth," she's a career oriented city planner. As "Liz," she's a more carefree teacher married to the handsome U.S. Army physician she met at the park. Scenes are cast in red or blue depending on the interwoven storylines of the moment.

It's a demanding assignment that keeps Menzel on stage virtually throughout the energetic show - great news for auds since the artist's dexterity and earthy voice are fully exploited by Kitt's melodic score. The task begins immediately with a lengthy opening chorus number punctuated by Menzel's vibrant entrance singing the signature tune "If." It ends almost three hours later with her socko number "Always Starting Over."

Offering full-throated support is a versatile team of Broadway regulars that includes Anthony Rapp ("Rent") as the long-time activist chum, James Snyder ("Cry-Baby") as the physician, Jerry Dixon ("Once on this Island") as the overly interested boss, LaChanze ("The Color Purple") as the spunky neighbor, Jenn Colella ("Urban Cowboy") as the neighbor's lesbian partner and Jason Tam ("Lysistrata Jones") as the spouse of Rapp's character.

Combined, they portray the motivated denizens of a truly hip New York City where folks are as likely to be gay as straight, almost everyone is self-absorbed and the F-word is a linguistic mainstay, as in the acerbic act one number "What the Fuck?"

Kitt's score adheres to the modern musical style of pop, rock and country themes embellished with soaring melodies and tight ensemble harmonies. The principals all make the most of their opportunities. Rapp serves up an act one highlight with the lively tune "Ain't No Manhattan," which also features some of Yorkey's cleverest lyrics. Snyder's pleasing tenor voice is displayed in "Here I Go," a country-flavored duet with Menzel, while LaChanze and Colella double the impact of their impressive talents in another country-influenced tune, "You Learn to Live Without."

It is all effectively staged on Mark Wendland's elaborate multi-tiered set complete with turntable, adjustable mirrored ceiling and countless scene variations to accommodate ever-changing locales. Director Michael Greif clearly does his best to keep the heavy load buoyant, as does choreographer Larry Keigwin.

Some unevenness is expected at this phase of any new show, and it's evident here in both music and lyrics of the tuner's 21 numbers. But the principal concern should be the book, which has laudable intentions but at times registers as tedious, inscrutable and predictable. Its many moveable parts are pursued at such a relentless pace that there is little opportunity for auds to take a breath. When a much-needed dose of sarcasm is finally inserted by the perky LaChanze character late in act one, it comes as a blast of fresh air.

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