The words “sugar, butter, flour” waft like a siren call in the opening moments of “Waitress,” the new Broadway-bound musical getting its out-of-town premiere in the test kitchen of the American Repertory Theater in Cambridge, Mass. From these basic ingredients, this musical’s heroine Jenna (Jessie Mueller, who won a Tony for “Beautiful”), an unhappily married waitress in a small-town diner, creates spectacular pies that are, as one character remarks, “biblically good.” And while this feminist fairy tale of a show, in which singer-songwriter Sara Bareilles makes an impressive legit bow as a composer, is quite delicious at times, it still needs more work — especially in the second act — before it’s a recipe for success.

Based on writer-director Adrienne Shelley’s 2007 indie film, Jessie Nelson’s book follows the movie’s narrative, beginning just as Jenna, in the midst of planning to leave her abusive husband, Earl (Joe Tippett), discovers she is pregnant. She decides to have the baby — but clearly it’s not a child she wants.

She finds support from her fellow waitresses, not dissimilar to the characters in the diner film “Alice Doesn’t Live Here Anymore” and its sitcom spinoff, “Alice.” There’s mousy, nervous Dawn (Jeanna De Waal) and tough, wisecracking Becky (Keala Settle). There’s also brusque Cal (Eric Anderson), the cook/manager, and the diner’s folksy-crusty owner, Joe (Dakin Matthews, in a performance as easygoing and tasty as molasses).

Things get more complicated when Jenna starts an affair with her newly-arrived-in-town — and married — gynecologist, Dr. Pomatter (Drew Gehling). As played by the sweetly awkward and disarmingly gentle Gehling, it’s the type of romance that, in another show, you would hope would work out somehow. But this distaff-centric tuner is not simply about getting Prince Charming, but also getting its heroine to clean up her own messes, take action and discover her maternal worth.

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