Kenneth Lonergan’s play is a painstaking, mostly unsentimental dissection of the frustration and sadness that comes from watching a once-vibrant woman descend into senility.

Gladys Green had been a vintage Greenwich Village bohemian lefty, the kind of woman people always call “a character.” As the play opens, traces of that former self are still visible, especially during Gladys’ visits to the small but lovingly tended Waverly Gallery, which she has run for decades at a small profit. But Gladys’ slide into senility starts to accelerate. Coates’ meticulously and maddeningly well-constructed Gladys becomes a fragile, querulous creature, given to repetitive, singsong monologues and endless questions. This is sadly punctuated by strange moments of clarity.

“The person that she used to be hadn’t really been around for a long time,” Daniel says.

Under the direction of John Tillinger, the play is at its best in scenes when the family is gathered together. Here there are moments of pathos, like layered monologues, in which Gladys drones on and on and the rest of the family continues holding a conversation right over them, and Gladys’ puzzled questions — “What’s everybody laughing at? What’s going on?” But there are also glimmers of strange comedy and tenderness in the ritualistic recitations of family mythology and Gladys’ surreally humorous non sequiturs.

The characters — and actors — show their mettle in these strained scenes. Altman’s bluff but well-meaning Howard shouts to make himself heard over the static of Gladys’ hearing aid and confusion. The self-contained, elegant Ellen — undoubtedly a lady who lunches — refuses to send her mother away to a nursing home. Daniel is eloquently guilty about his ambivalence toward the problem, admitting that he sneaks away from Gladys and makes his visits a bit “stingy.”

But as Gladys degenerates, she starts to lose everything, even words.

“Jesus Christ, it’s word salad,” Ellen says.

The family’s fabric degenerates, too, into a series of worried, miserable deliberations. What about getting a cat? Is she really safe alone? Is she getting worse?

Above all, the family is haunted by a sense of uncertainty.

“I don’t know what’s going to happen to her, but I wish it would just happen,” says Ellen, who worries that her mother may stick around, demented but relatively healthy, for all too long.

Struggling to balance “safe” and “tolerable,” the family is forced to pile up debts of gratitude to Don Bowman (Adam Trese), an aspiring artist who sleeps on a cot in the gallery’s back room and unwittingly becomes Gladys’ caretaker.

But the crisis comes when Gladys’ landlord, Alan George (Tim Donoghue) evicts her from the gallery to make room for a new cafe for his hotel. Under a thin veneer of sympathy, George is a hard-boiled businessman, impervious to the family’s pleas to allow Gladys just a little more time. With Gladys deprived of any stimulation or reason to leave her apartment, Ellen and Howard are forced to take her into their home and start waiting it out.

“Maybe we’ll all survive” is Ellen’s wry but weary comment.

Lonergan’s play isn’t perfect. Daniel’s explanatory monologues, spoken directly to the audience, are sometimes superfluous and overly sentimental. Occasionally, the minimalism of the plot and the thick and fast flow of agonizing moments slows the play down to a grim, grinding pace.

But Lonergan’s honestly written characters and ruthless realism make this play gripping and highly relevant. The tragedy of Gladys’ deterioration and the struggles of her family resonate deeply.

There is no moment of enlightenment here — and there can’t be. This is a situation whose only “solution” is Gladys’ death, and Lonergan rightly won’t let you forget it.