Call her plucky, that “Girl of the Golden West.” And starry-eyed. And true blue. Girl, as the lovable miners of her Polka Saloon call her, is all that and cute as a button.

TheatreWorks presents playwright David Belasco’s 1905 ode to love amid the California gold rush in the final production of its 40th season. “Girl” runs through May 15.

Belasco, a Broadway powerhouse who was born in the Bay Area during the gold rush, amassed all the elements in “Girl” that a melodrama requires: a rowdy saloon filled with hard-drinking miners, bandits and shootouts, a girl with a heart of gold, a love triangle and a suitor with a questionable past.

Knowing the elements, you can probably guess the story, which plays out against Russell Parkman’s meticulously dressed and amazingly flexible set. Suffice it to say that the course of true love jumps the track by the middle of Act II.

And while Belasco’s reported bent toward naturalism makes strong appearances in the performances by the leads – Caitlin Wise as the Girl, Michael Gonring as bad boy Dick Johnson and Jon Barker as Jack Rance, the town’s dandified sheriff and rival for the Girl’s affections – it is ultimately fractured by the weight of predictability.

Do you need to believe all of the action in a little bon-bon like this? No, not really, but when you combine stock characters and situations, dime novel dialogue and a long running time (2 hours and 45 minutes, including two intermissions), even charm and strong ensemble acting can’t buoy “Girl” for an entire evening.

The most consistent performances on opening weekend came from the supporting cast, which includes standouts Robert Rais (mysteriously billed as Bernie Kluszewski), Danny Bristol, Mark E. Cannon and Emory John Collinson. As the dwarves to Wise’s Snow White, they argued and blustered, blushed and shuffled while also suggesting real human beings. Andy Sturt as the bartender and Bruce Carter in a small role as the Wells Fargo man instilled real dimension in their small roles

The problem facing the leads, though, was tough: Balance the real with the archetypal in a way that elevates the penny dreadful plot, thus engaging the audience. The often purple flourishes of turn-of-the-century dialogue didn’t help matters.

“Lying out there in the clay,” Girl says of a dying miner, “I guess the cleanest thing about him was his soul.”

(You can perhaps understand why Giacomo Puccini adapted “The Girl of the Golden West” for his 1910 opera “La fanciulla del west.”)

Wise, who was hypnotic in TheatreWorks’ 2015 production of “The Liar,” does her best with many long monologues. She’s clearly crafted a specific country accent for the Girl, which is consistent, but hard to place. Like all the leads, she strikes the most convincing and captivating balance in the melodrama’s genial first act. As the play churns on, though, Wise is unable to strike that balance and sell the plot turns of Act II, resulting in a Girl who is somewhat less sympathetic by the end of the play.

Ditto for Gonring and Barker, able actors trying to walk that same thin line.