"Wonder of the World," which opened over the weekend at Woolly Mammoth Theatre, is one of those zany plays with a sad center that Woolly specializes in. David Lindsay-Abaire's comedy about a runaway wife who befriends a woman planning to go over Niagara Falls in a barrel makes for a milder evening than the premise suggests, but it's vibrantly directed by Tom Prewitt and has a top-flight cast.

Cass Harris (Deb Gottesman) has fled her nerdy husband, Kip (Michael Russotto, scarily successful as a dork), after discovering he has a terrible sexual secret. And believe me, even in this era of perverse confessions, Kip's little quirk is a zinger. Any sane woman would run, run while she can! Unfortunately, Cass stops in Niagara Falls, where a couple of amateur private detectives (out-of-it Bruce Nelson and fumingly impatient Kerri Rambow) catch up with her. She also finds herself lending emotional support to a distraught would-be suicide who has come to the falls lugging her own barrel.

As played by Gottesman, Cass is spacy and spunky, a cockeyed optimist. Psychologically, Gottesman always seems to be at a bit of a tilt, as if Cass were expectantly leaning toward what's sure to be a better future. Or maybe she's just falling over, tripped by the circumstances she thinks she's escaping.

Nancy Robinette is the barrel rider, Lois, abandoned by her husband, suicidal and drunk. Always remarkable, Robinette is really something special here: ethereal yet earthy, angry and faintly tragic. Lois's rage protects her from the knowledge that life is too much for her--and yet Robinette's distracted air suggests that, over the noise of her defenses, Lois hears the cold whisper of oncoming defeat.

Cass has made a list of things she has always wanted to do and is busily checking them off. One of the major items is Ask Big Questions. Her biggest question, and the play's, is how much of life is fated. Running from her weird marriage, heading for freedom, Cass soon finds herself emotionally entangled again and surrounded by a new kind of weirdness. Has she gotten anywhere at all?

Though it contains a lot of explosive comic action, "Wonder of the World" doesn't blow up into wild chaos. Its movement is dramatically quiet, as life's strangeness and sorrow reestablish their hold on Cass and pull her back to them.

Lewis Folden has designed areas on the edge of the acting space that are tiny museum exhibits about Niagara, complete with old books and postcards. This funny, sweetly nostalgic touch is perfect for the play's sadder resonances, but this modestly conceived set is overwhelmed when things go berserk.

For most of the evening, Prewitt keeps the production in a graceful comic whirl. The play is funny and very likable, and he brings out its best elements. But the big nutso climax, though designed as lethal farce, goes by without much impact. It's as if Lindsay-Abaire, who seems fundamentally a gentle writer, had lost his conviction about the scene. The play loses its energy after this, and drifts rather than roars to a close.

Wonder of the World, by David Lindsay-Abaire. Directed by Tom Prewitt. Lighting, Lisa L. Ogonowski; sound, Hana Sellers; costumes, Justine Scherer; props, Linda S. Evans. With Kirk Jackson. At Woolly Mammoth Theatre through July 2. Call 703-218-6500.