In the last act of Arthur Schnitzler's "Undiscovered Country," an amoral husband is asked why he has just fought an essentially pointless duel and killed an inconsequential rival. It was not out of honor or vengeance or even a respect for old forms. Why then?

"Obviously," he says, fumbling for an answer that is not there, "because I felt like it."

He is not alone. In a play that is both prickly with cynicism and generous with compassion, Schnitzler is dissecting a whole society--Vienna in 1911--that can come up with no better explanation for the way it trifles with love and morality. Human behavior is a country without maps. Motivations are too entwined, too deeply buried, too mocking to submit to rational analysis. As one of Schnitzler's wiser characters observes, "We try to bring order into our lives as best we can, but that very order has something unnatural about it. The natural condition is chaos."

At Arena Stage, which revived the 71-year-old Austrian drama last night, the chaos is sad, sometimes silly, and ultimately curiously gripping. Schnitzler's elegant characters skitter along the surface of life, devoting their days to dalliance and tennis, but the night is falling rapidly. The trivial acts accumulate and the thoughtless remarks multiply. Then all of a sudden the brutal consequences come home to roost and no one quite knows why. Superficial people have acquired the moving resonance of the bewildered.

Not surprisingly, Sigmund Freud, one of Schnitzler's contemporaries and a fellow believer in the dark tangle of the soul, greatly admired the playwright's work. So does Tom Stoppard, the British dramatist who is responsible for the alert adaptation that Arena is using. In his own plays, Stoppard often examined that mischievousness of language, as if words had a life of their own, and delighted perversely in leading the speaker far afield. That, as it turns out, is not too far removed from Schnitzler's concern: man's unflagging capacity to delude himself in love and boredom.

"Undiscovered Country" is precisely the kind of play for which Arena has always shown a special affinity--a sweeping social panorama (the cast numbers 28), essentially realistic in detail, but brushed with a poetic melancholy. Set in the garden of a country villa, then in a posh mountain hotel, and finally a parlor, it is focused on two world-weary characters: Friedrich Hofreiter (Richard Bauer), a prosperous manufacturer of incandescent light bulbs, and his gracious wife (Barbara Andres), whose handsome exterior reflects her concern with appearances in general.

Unknowingly, she may have contributed to the suicide of a pianist before the play begins ("an artistic tantrum" is how one of Schnitzler's social moths dismisses that death). Knowingly, he retaliates by courting a strawberry-fresh neighbor (Julie Osburn). The ball is back in the wife's court; she drifts into an affair with a headstrong naval lieutenant (Robert Westenberg). Like little waves in a warm pond, the casual immorality ripples outward to embrace friends and acquaintances. No one is evil, but only a socially maladroit doctor (Stanley Anderson) has the spine to resist.

Garland Wright has directed the drift of affairs with confident leisure and grace, never forcing these glib characters into harsh drama before their time. Except for a momentary dip, when the action moves, mid-play, to the mountaintop, Arena's production gathers density in subtle and fascinating ways, so that the audience is enfolded by drama almost without knowing it.

Bauer's richly satisfying performance moves back and forth over a wide arc, delineated on one end by the snorts of laughter he invariably tries to quash with his hand, on the other by a quiet knot of anguish that stills his exuberant features. Andres, a woman of imposing beauty, deftly suggests all the turbulence in the wife's soul by resisting it with as much dignity as she can muster. Westenberg, Anderson, Osburn and Charles Janz, flibbertigibbet at the tea party, are equally strong presences in what appears to be less a cast of actors than an actual swatch of the social fabric at large.

To no one's especial concern, that fabric is slowly coming apart in "Undiscovered Country." Then it is too late. Wright stages the final act in a harsh shaft of light that seems to be spilling into the parlor through an open doorway. Two high-backed chairs are the only furniture. As the characters grapple with the fruits of their actions, they cast fleeting shadows on one another's faces. It's an arresting turnabout for a play that started out in the dapple of summer sunshine.

UNDISCOVERED COUNTRY, By Arthur Schnitzler. Adapted by Tom Stoppard. Directed by Garland Wright; sets, Adrianne Lobel; costumes, Marjorie Slaiman; lighting, Arden Fingerhut. With Richard Bauer, Barbara Andres, Julie Osburn, Robert Westenberg, Stanley Anderson, Charles Jansz, Halo Wines, Mark Hammer, John Madden Towey. At Arena Stage through April 18.