INMAN SQUARE HAS BECOME a sort of flower-case cultural center in Cambridge. Like an old gem in an awfully tacky setting, this confluence of streets dimly shines out of the decaying sprawl of industrial and residential space that is East Cambridge. In recent years, Inman Square has become the site of several good bars, some firstclass restaurants and a dinner theater. Last Wednesday the venerable intersection established further claim to its position in Cambridge's cultural firmament with the opening of the Off-Broadway Theatre on Hampshire Street, in the garage-like structure that was the home of The Proposition until last summer. The Off-Broadway Theatre is a professional company that plans short runs of a wide variety of theatrical genres at current rates, which means fairly expensive.

If last Wednesday's gala opening-night crowd seemed a bit too self-consciously arty, even for Cambridge, it was probably because of the unusually large number of reviewers and other theater-types invited to witness the first (snow-delayed) performance of the new company. But the nature of the audience itself will change; what is important is that people support this fledgling institution. Too many starving actors, you know.

Any worthy theatrical venture merits sincere wishes for success, but the Off-Broadway Theatre may not get much of a chance to show its stuff if it doesn't come up with better material than American Buffalo, the David Mamet play of several years ago chosen as its first production. These days, Mamet is a playwright of some note. His talent seems to have matured--his A Life in the Theatre, currently running at the real off-Broadway, in Greenwich Village, is a marvelous work, full of wit and the kind of charm only a developed writer can muster. But American Buffalo is clearly from Mamet's earlier, less developed phase. Walter Kerr succinctly described it as "a play in search of a plot." In the case of this production, a solid cast is largely wasted in a fruitless theatrical exercise.

American Buffalo is not a total loss, of course; Mamet is too talented for that to be the case, and director Tom Bloom squeezes everything he can out of the book, turning out a solid first act. The action--or lack thereof--takes place in a secondhand shop in a city, later revealed as Chicago in a passing reference. Three characters complete the cast, and everything transpires in the shop itself, elaborately designed and filled with junk props. The Off-Broadway's technical crew must be a good one, with careful attention paid to minute details like the drab, industrial green paint on the walls and even a dripping faucet.

THE PLOT REVOLVES around Donny Dubrow, the proprietor of the resale ship, seemingly a one-time street punk now in his late 20s, eking out a marginal existence. Donny spends the opening minutes of the play expounding his philosophy of life and business to Bobby, a nervous, denim-clad teenager who serves as Donny's sometime-assistant and partner in petty crime. Donny's theory is somewhat simplistic, summarized in the phrase "Action talks and bullshit walks." The point of this diatribe seems to be that everyone must look out for themselves. Stuart Burney's Donny seems painfully aware of this maxim, finding it distasteful, perhaps, but true. Burney lends an air of realism to his character; his Bonny is like thousands of backstreets city kids aware of the odds against them and unable to score anything more than marginal victories against the system that keeps them in place.

Paul Guilfoyle's Teach provides the moving force in the play. Both Donny and Bobby (jitteringly played by Lloyd Brass) are deferential to Teach, a self-assured, macho punk and Donny's old buddy. Guilfoyle brings an excellent manic intensity to his part. His mannerisms, a shambling set of neverending words and motions, are largely reminiscent of Robert De Niro in Mean Streets. Teach, a grown-up yet immature punk, follows De Niro's Johnny Boy, save that Guilfoyle lacks De Niro's genius, and his Teach is self-consciously smart, whereas Johnny Boy is too dumb to know any better.

What little action there is concerns Donny's attempt, masterminded by Teach, to steal a valuable buffalo-head nickel he sold to a customer several days before, unaware of the coin's worth. Teach plans the crime, infecting Donny with his enthusiasm and such telling logic as "You make your own right and wrong...so you know what I'm talking about here, huh?" Finally, Teach and Donny plan the caper, and Bobby is replaced by an absent friend named Fletch, at Teach's insistence.

So ends the first act, with the audience expecting some kind of real action, at last, in the second act. The tensions are there--the plotted crime, as well as growing differences between Teach and Donny, mostly concerning Bobby, but also inspired by Teach's self-assured abrasiveness. But nothing really happens until the end of the second act. Instead, Mamet gives us his version of Becket, perhaps entitled "En Attendant Fletcher." The act limps by as the characters wait for the arrival of their accomplice, and the tensions between them continue to build. Mamet, it seems, wanted to show the ultimate powerlessness and futility of his characters, like Vladimir and Estragon in Waiting for Godot, but without the existential mantle that cloaks Becket's fine work and gives it its legitimacy, Mamet simply cannot pull it off. There is just not enough of a plot to give his idea any weight, and what little there is is far too flimsy to bear the tensions Mamet imposes. A tacked-on existential conclusion to a mundane drama about the values of the urban lower class is just too much of a contrast. Had Mamet stuck to one theme or another, the conclusion might have made more sense.

AS IT IS, the tensions between the men build as time passes. When Teach brandishes a revolver, the audience feels he must surely use it, but this is a bit of deceptive foreshadowing. Instead, Teach goes berserk, provoked by Bobby's apparent lies. He brains the poor kid with a lamp and then proceeds to trash the set in a fine display of uncontrolled rage. This moment of Brando-esque pique is genuinely frightening, but somewhat inexplicable. Just as suddenly as he began, Teach stops, becoming apologetic. With all his bravado dissipated, he becomes pitiful...but why? The motivations remain cloudy, and so the ending, which features a confused Donny cradling the slightly dented Bobby in a touching Pieta pose, is ultimately unsatisfying and vaguely weird.

American Buffalo is a baffling play, and not a very good one, either. If Mamet meant to sketch the mean materialism inspired among the lower fringes of a capitalist society, he could have succeeded. But his play stretches for some deeper meaning, and in reaching for that goal topples over, incomplete. It's a shame to waste three fine performances on such a strange play. But the general strength of the production, if not of the play itself, is encouraging, and it is good to see an energetic young professional company set up shop in Cambridge.