American playwright Rachel Crothers isn’t talked about much these days, but between 1906 and 1937, nearly 30 of her plays were produced on Broadway. The last – and possibly the best – was “Susan and God,” a 1937 drawing-room comedy of surprising depths. Now, in Jonathan Bank’s first-rate revival at the Mint, “Susan and God” emerges as a rediscovered American classic, a powerfully affecting story about how difficult it is – indeed, how nigh-impossible – for a person to really change.

The mania for self-improvement sweeps into a country-house drawing room with the stylish high-spirited Susan (Leslie Hendrix). Standing in the midst of her sophisticated friends in a fetching red outfit, she tosses her long auburn mane and chatters nonstop. Susan is fresh off the boat from England, where one Lady Wiggam took her under her wing and introduced her to “the most remarkable collection of people,” who confessed their failings to each other and asked God to help them change. “It’s just love, love, love,” she explains earnestly, as her skeptical friends dissolve in giggles.”It’s thrilling – and alive and fun, so people aren’t ashamed to be good.”

Having seen the light, Susan is determined to rip off her friends’ blinders. They must confess! she declares, confess before one another. And the talented ensemble cast plays off Susan’s melodrama with terrific flair. Susan’s friends are accustomed to her long,dramatic speeches; usually they find them amusing. But this time she’s gone too far.When she gallantly strides in all her glory across the parlor to tell a man he must declare his love for a married woman, the atmosphere prickles: She’s so rude! She’s so pushy! She’s so fanatical! … And she’s so right!

By the end of the evening, Susan’s holier-than-thou act is no longer funny; it’s just hypocritical. While she urges everyone else to confess, her own confession (she touches up her gray roots) conveniently omits the fact that she has essentially no relationship with her alcoholic husband or her abandoned teenage daughter. Finally, at the height of her proselytizing, her husband Barrie (Timothy Deenihan) walks into the room.The party’s over.

So ends the first of three expertlyconstructed acts. Ms. Crothers clearly mastered the arts of pacing and dramatic tension, and there is no wasted space in “Susan and God,” just sharp, lifelike dialogue laid out in a series of well-timed arcs. This is fine-tuned, masterful writing, that earns both its laughs and its sharp kicks to the gut.

It’s a brilliant stroke that the inebriated Barrie is the only person in the room who takes Susan seriously. Moved to tears, he asks his wife to reform him. Susan is visibly appalled, but refusing him would make her appear a fool in front of her friends – and besides, taking him in would be self-sacrifice worthy of the beatific Lady Wiggam herself.

So begins the painful experiment at the heart of “Susan and God,” in which Barrie tries to quit drinking, and Susan tries to be a wife and mother. As visibly hard as it is for Barrie to fight his addiction, it may be harder still for Susan to rewire her emotional impulses. When Susan’s daughter Blossom (Jennifer Blood) hugs her, she instinctually recoils, and she even forbids Barrie to touch her.

The character of Susan – both in Ms. Crother’s script and in Ms. Hendrix’s rich portrayal – renders a high level of honesty. Susan, “a woman with no thought in life except what will be most pleasant for Susan,” finds it positively excruciating when the world doesn’t accommodate itself to her wishes.(And who can’t relate to that sensation?) The charming superficiality that sweeps her along through life is also a defensive posture – she would like, after all, to have a husband, but Barrie has disappointed her too many times.

Susan’s faults are keenly familiar, and her struggle to improve herself – as misguided, incompetent, and lazy as it often is – is moving. All at once it becomes apparent that Barrie’s touchand-go sobriety depends largely on a wife who may no longer love him. From drawing-room comedy, “Susan and God” gently shifts into the murky, terrifying territory of the human heart.

Somehow, even as it pokes fun at Lady Wiggam’s movement and Susan’s rapid conversion, “Susan and God” casts its vote for the untapped power of goodness. Could Susan’s foolishness be in some part wisdom – as when (in 1937), she gushes that practicing real love for other people “is the only thing in the world that will stop war”? By the final riveting scene, when Susan’s mettle is tested once and for all, we sit on pins-and-needles, hoping against hope that she will find some goodness in her pinched, disused heart. And when her skinny arm reaches out to bestow a shaky caress – its first in years – it’s hard not to believe in the power of goodness to change the world, one poor wretch at a time.

Until July 16 (311 W. 43rd Street, between Eighth and Ninth Avenues, 212-315-0231).