There is a man climbing the tree outside Alexandra's window. The 79-year-old artist looks up from her easy chair and shrieks with recognition, "Chris!"

It is indeed Chris, Alexandra's son, and it turns out that she has not seen him in 20 years. So might there have been a moment of apprehension, a bit of confusion in her failing mind, an amazement that her son is crawling up the tree outside her Brooklyn brownstone?

Nope. She tears into him like he was just sneaking in after staying out all night.

It's small stuff like that — and the large stuff — that diminishes Eric Coble's "The Velocity of Autumn," which opened unsteadily in Kent Knutson's production Saturday at the Old Log Theatre.

Coble chose a worthwhile cliché and one that many in the audience certainly are dealing with: how to manage a parent's decline. We lose sight of those good intentions, however, in dialogue that only occasionally penetrates the formula and an air that conjures a "Maude" reunion more than an honestly emotional show.

Melissa Hart plays Alexandra, walking about in her spacious and beautifully appointed living room, with set design by Erik Paulson. Outside, the large tree that is Chris' entry point is aflame with autumn color.

Chris (Paul de Cordova) has returned from self-imposed exile to try to save his mother. His siblings want to park her in a nursing home. She's barricaded herself with a Molotov cocktail in one hand and her father's Zippo lighter in the other. If anyone tries to take her, she'll burn the place down.

Mother and son talk, yell, argue, find their commonalities. She likes him best because he's an artistic spirit like her. Hart is quite affecting as Alexandra muses on the fear of becoming useless in her dotage. That poignant admission, and Chris's epiphany about life's impermanence in the fragility of sand art being swept away, reward our patience.

These moments of grace, though, surface rarely in a murky script that churns the same material over and over. Coble manipulates his characters to say what he wants them to say.

De Cordova finds in Chris a loner who has run from the family, feeling ostracized because he's gay — which his mother insists is not true. De Cordova makes a convincing case that Chris is a hero — a bit neurotic but a good guy.

Anyone who has acted has had that horrible dream of being backstage opening night and realizing you haven't looked at the script. Hart lived a mild version of that Saturday, needing to bring some pages on stage for reference.

It was awkward, but what an incredible acting performance by someone who finds herself in a tough position. If only Alexandra's story, told by Coble, had been as heroic in its reality as was Hart's performance.

If you are moved by sentiment and the subject, "Velocity" might be worth the 90 minutes. But I'll bet "Maude" could get it done in 30 and say much the same thing.