The Broadway comedy (you remember the Broadway comedy) has become such an endangered species that one should probably applaud Andrew Bergman's "Social Security" just for showing up at the National Theatre, where it began a three-week tryout Saturday.

Actually, you can applaud it on several other counts, too. It boasts some very funny lines, a personable cast headed by Marlo Thomas and Ron Silver, and a handsome apartment set worthy of Metropolitan Home. It may not be a particularly wise comedy, but it has an awareness of human idiosyncrasy that has been neatly exploited by director Mike Nichols.

What it can't lay claim to, however, is much of a plot. The lives of Bergman's characters are certainly in a fair amount of flux. But what the author tends to do in each of the play's three scenes is describe an awkward situation. Once he's filled us in on all the particulars, he rings down the curtain. "Social Security" turns out to be a kind of ongoing progress report on events, which for the most part occur offstage between the scenes.

Two sophisticated New York art dealers -- David (Silver) and his wife Barbara (Thomas) -- are awaiting with some foreboding the arrival of Barbara's sister and brother-in-law, Trudy and Martin. Trudy (Joanna Gleason) and Martin (Kenneth Welsh) live in Mineola and are about as exciting as a shopping center parking lot. He's a mousy accountant; she's a tight housewife, for whom the purchase of a wedge of Brie represents an unconscionable extravagance.

They have a problem: Their 18-year-old daughter, it appears, has discovered sex during her first year of college in Buffalo. She told her father so over the phone. "I live for sex. Everything is sex," she said. (He copied down her words verbatim -- along with some considerably hardier details -- and carries the slip of paper about in his wallet.) Trudy and Martin have decided to reclaim the wayward child, which brings us -- about a half hour into the show -- to the real object of their stopover in New York.

They're dropping off Sophie (Olympia Dukakis), Trudy and Barbara's aged mother, "like a package from Altman's." (She's sitting sullenly in the car, and the apartment doorman has been tipped a lavish two bucks to keep his eye on her.) Now David and Barbara must face the daunting prospect of caring for her.

And daunting it is. Sophie, you see, clumps around on a walker, leaves half-sucked sourballs all over the house, feigns deafness when it suits her, and just last month ran up a $300 phone bill because she couldn't manage to get her area codes straight. At the end of the act, Sophie, hair astraggle and a glum expression on her face, appears at the apartment door.

Okay, we're off to a reasonably promising start. But in Act 2, we're obliged to begin anew. This time, David and Barbara are preparing to entertain a celebrated 100-year-old artist (Stefan Schnabel). They're praying that Sophie won't make too great a spectacle of herself. They'd be happy, in fact, if she put on a dress for the occasion. Over the cold mousse of pike (gefilte fish apparently isn't "festive" enough), Sophie surprises them by getting real cozy with the artist. Before long -- my heavens -- she's even flirting and he's beaming like a schoolboy. Curtain!

The last scene, well, consists of even more catching up. By this time, Sophie has undergone a total transformation. She's having an affair with the centenarian, wearing stylish Adolfo suits and gearing up for a trip to France. Meanwhile, Trudy and Martin are back from Buffalo, where it seems . . . Well, you get the idea. "Social Security" is a play in which the characters are forever telling one another what happened to them when they weren't on stage.

Bergman, who comes to the theater from films ("Blazing Saddles," "Fletch"), has a keen eye for the absurd details of the contemporary scene and he's drawn some savory contrasts between his trendy Manhattan couple and the two dowdies from Mineola. Gleason very nearly runs away with the show with her portrayal of the pinched housewife, who is suffering a quiet martyrdom -- that is, when she's not correcting her husband, complaining about excessive prices or dredging up childhood rancors.

Amusing as these various accounts of woe can be, though, they don't quite satisfy our hunger. After a while, we'd like to witness things firsthand. Yet Bergman deprives us of Sophie's change from arthritic frump to incipient jet-setter (which may be why the character seems the least credible). Trudy and Martin find their lives upended, but that transpires on the way back from Buffalo. As for David and Barbara, they don't really ever get to do anything other than squirm over the disruption of their fashionable life style.

Admittedly, Thomas squirms very nicely -- her gamin charm undimmed by time, her scratchy voice as bewitching as ever. Silver, who gets to shoot off some of Bergman's better cracks, is an affable comedian -- shaggy enough so as not to appear glib, smooth enough to avoid the charge of being a wise guy. Nice performers, both, but they're definitely underemployed.

Under Nichols' alert eye, "Social Security" has the polish and professionalism you expect of a Broadway comedy. (You do remember the Broadway comedy, don't you?) But watching its three scenes, you are apt to conclude, is rather like watching three distinct episodes of a television sitcom.

Social Security, by Andrew Bergman. Directed by Mike Nichols. Sets, Tony Walton; costumes, Ann Roth; lighting, Marilyn Rennagel. With Marlo Thomas, Ron Silver, Joanna Gleason, Kenneth Welsh, Olympia Dukakis, Stefan Schnabel. At the National Theatre through March 22.