What might one day be a trenchant black comedy about an aging parent and an alienated child is—in its current incarnation at Arena Stage—two or three fine soliloquies connected by tissue of wanly familiar relationship drama and a dubious conceit. What makes this startling is the volume at which Eric Coble’s big-hearted but uneven one-act The Velocity of Autumn has been noised about as Broadway-ready. That, it sincerely ain’t.

It is, at present, a shaky showcase for two prodigious talents who seem to be still feeling their way around the contours of Coble’s script, which finds a dyspeptic aging artist, Alexandra, barricaded inside the Park Slope brownstone where she and her late husband once raised the family that now wants her out of the place. A danger to herself, she might be, and to others. Certainly she’s willing to threaten to be the latter: At lights-up, she’s sitting in her favorite chair, surrounded by a few dozen homemade Molotov cocktails, having threatened to take the place down to the foundations if the kids don’t leave her in peace.

Into this already unlikely scene comes not a SWAT team or a sniper, as you might reasonably expect in a post-9/11 New York City, but a long-absent son. Chris makes his entrance via the second story, climbing Mom’s favorite tree to access the window he knows she never locks—because reasons, and sentiment, and authorial shorthand for the special connection these two odd birds shared among their larger domestic flock. Having long since fled for the less-pressurized climes of the American West, Chris has been deputized by the sister he still speaks to and the bully of a brother he loathes to talk Mom down.

And man, do they talk. Loudly, tenderly, fondly, abusively, and at length, they talk. Coble’s play is such a writer’s exercise that even several days after the show’s press night, the estimable Estelle Parsons—an Oscar winner for Bonnie and Clyde and beloved by my generation as the title character’s insufferable mother in Roseanne—still seemed to be flailing a bit to keep her lines in order. “But she’s 85,” one might demur. “Cut her some slack.” Done, but the show’s other star, two-time Tony winner Stephen Spinella, also seemed a little bit at sea. And quite frankly, even if that was just acting—one of the characters is showing signs of early dementia, after all—Velocity is still wordy enough that it’s able to cram all the character and situation development I’ve laid out above, plus more, into 95 minutes and still find time to be earnestly, wearingly repetitive.

Those tent-pole speeches, though: You can see why the assembled talents believe in this play. Parsons settles authoritatively and with grace into a long rhapsody on how the experience of life is like the outings she and her son used to take to the Guggenheim Museum, on how—as with the art that approaches and recedes as a gallerygoer walks that spiraling ramp—our ever-changing perspective inflects and informs our understanding of the events and people that come our way, but never stay entirely still. Spinella—he was Prior Walter in the Broadway premiere of the Angels in America plays, so he’s no stranger to heated passions wrapped up in ecstatic arias—does shivery, profoundly anchored justice to an equally hefty moment involving a hideous memory and a sobering arrival at a new measure of self-awareness. Those speeches are lovely things and beautifully handled; they are not, alas, enough to make The Velocity of Autumn feel, for now, like more than a good idea on the way to its eventual form.