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Dark, twisted, and often beautiful, Passion, going on now until September 23, reaffirms Signature Theatre’s commitment to the works of Stephen Sondheim, as well as Sondheim’s commitment to musicals made for and by adults.

As directed by Matthew Gardiner, Passion is one of those shows that forgoes a lot of emotional range in favor of homing in on a particular theme, putting all its chips and spending all its time, in this instance, on an exploration of romantic obsession. It’s all about what love is, and, just as importantly, what it isn’t.

Giorgio (Claybourne Elder) is a young captain in the Italian military sometime in the mid-19th century. He’s happily having an affair in Milan with a beautiful blond woman named Clara (Steffanie Leigh gets to wear all but one of the show’s most fun costumes, designed by Robert Perdziola), but they are separated when he’s reassigned to a remote, desolate village with little in the way of scenery or entertainment. Life here is hell, the men sing, bemoaning the brown landscape, terrible food, and boring conditions.

And hell has a particular torment in mind for Giorgio in the form of Fosca (Natascia Diaz), the female cousin of his commanding officer. Fosca, who dresses all in black and is ill in that Victorian disease-with-no-name kind of way, immediately falls for the strapping, sensitive captain, and, well, the short version is she doesn’t know how to take “no” for an answer.

That’s putting it mildly. Fosca looks and acts like she wandered in from a Brontë novel — down to the imperious black hairdo. She latches onto Giorgio like a shadow; her “seduction” is so ingratiating, so artless and humiliating, that makes audience members squirm. If she were a man, you’d call her a sexual predator, pure and simple.

And yet… even creepy, shameless Fosca has her charms. And she might have something about love to teach Giorgio and Clara.

This production, strong in both performance and in every technical regard, might itself have less to teach, however. The landscape may be all brown, but James Lapine’s book could itself use a few more colors. And that’s hardly a dig at Lee Savage’s clever set design or Collin K. Bills’s exquisite lights. It’s tough to imagine a better Passion, which is exactly the problem.

Diaz, Elder, and Leigh all have terrific voices and acting chops to match. Diaz has to deliver soaring arias while both sitting and lying down — way harder than it looks — and she wrings every drop of humanity she can from a wholly unsympathetic character. Elder brings to mind Hitchcock’s famous compliment of Grace Kelly: he’s a snow-covered volcano. Staid and chilly for much of this one-act musical’s two-hour run time, his eruptions count all the more. And Leigh turns what could have been a thin character, appearing on balconies to sing her letters, into a fully realized one.

But some humor or wit to lighten (or just vary!) the mood would have gone a long way. Hell, even Sweeney Todd has lots of laughs in it. It might be cruel to call this a top-shelf production of a second-tier Sondheim, but the truth, like romantic love, is sometimes cruel.

After the curtain call, my theatre companion, a Hamilton devotee, quipped that Passion was now her “second favorite musical that ends with a duel.” Clearly, she missed Signature’s production of A Little Night Music.

Passion runs at Signature Theatre through September 23