Kirsten Johnston, second from left, and Michael T. Weiss, right, star in 'Scarcity,' Lucy Thurber's sensitive portrayal of life on the wrong side of the tracks, directed by Jackson Gay.

Kristen Johnston ("The Lights," "3rd Rock From the Sun") heads up the well-oiled ensemble as Martha, a hard-working woman who survives on sheer grit, cheerfully holding down a slave-wage job at the local mall to keep her family intact and their heads above the poverty line.

As enthusiastic about marital sex (and the brawls that precede it) as she is sloppy about housekeeping, Martha is the sort of woman who shares her cigarettes with her 16-year-old son and tosses a beer to her husband after bailing him out of jail.

In a sitcom, Martha would be written and played for mean laughs. This complex character gets a much fairer shake from Thurber, who perceptively views her as an uneducated but intelligent woman who knows what's best for her family and makes it happen -- no matter the personal cost.

And in Johnston's acutely sympathetic portrayal, this earthy wife and understanding mother acquires a vital physical presence, coming alive as a really great broad with a lot of heart.

Of the comically clueless adults who orbit Martha's light-giving star, no one has an inkling of the good/bad changes that she can sense coming around the bend. Certainly not her quarrelsome cousin, Louie (Todd Weeks), who is forever bursting through the door -- often pursued by his slatternly wife, Gloria (Miriam Shor) -- in the vain hope of jumping her bones.

And surely not Martha's worthless husband, Herb (Michael T. Weiss), a slovenly if still sexy wreck of a man in Weiss' brawny (and subversively brainy) performance, who drinks when he's happy and drinks when he's sad and clings to his wife and kids for confirmation of his essential worth.

Herb may be dumb and dangerously prone to violence, but as Weiss reads his fogged-up mind in flashes of startling insight, he's not stupid or insensitive. Like those inarticulate lower-class characters in Beatles song narratives, the big dope can even be eloquent. "You remember me, don't you?" he asks Martha, in a quiet, beautifully written moment between fights and sex. "I'm counting on you. I might as well be dead, otherwise."

Martha's response -- "I miss you more than you will ever know" -- is every bit as devastating.

Although it's normal to expect any kids of this turbulent union to be carbon copies of their elders, Martha and Herb have produced two preternaturally smart children who are protective of their parents in the way that children who grow up in dysfunctional households tend to be.

These kids are not only smart, they're clever. Sneaking in and out of the house like a thief, Billy (Jesse Eisenberg in a tightly coiled perf) is secretly plotting his escape by seducing Miss Roberts (Maggie Kiley), a young teacher with the connections to get him a scholarship to Deerfield Academy. In Billy's desperation to get away from home, he is abandoning Rachel (Meredith Brandt), his much brainier but defenseless 11-year-old sister.

As young Brandt plays her -- with fierce intelligence behind a facade of little-girl goofiness -- Rachel is way ahead of everyone in the family, but unable to be a proper caretaker for any one, including herself. And with Billy running out on her, her best chance for survival is to make an end run at Miss Roberts herself.

Thurber writes with both humor and pathos about this household, whose family values of love and loyalty are constantly put to the test in an environment of poverty, ignorance and casual violence.

Behind the snappy dialogue and brazenly comic characterizations, she also shows genuine tenderness toward people who rarely get that kind of treatment on the stage. And that's all reason enough to peg this scribe as a keeper.

Sets, Walt Spangler; costumes Ilona Somogyi; lighting, Jason Lyons; sound, Daniel Baker; original music, Jason Mills; production stage manager, Marion Friedman. Opened Sept. 20, 2007. Reviewed Sept. 13. Running time: 1 HOUR, 50 MIN.

© 2005 Reed Business Information, a division of Reed Elsevier Inc.