Behind the Mattress Factory garden rises a towering stone wall clad in green ivy; below, crumbling foundations. A street light casts a sharp, oblique shadow, while viewers crowd the foundation tops and steeply banked seating.

	"When the World Was Green" Where: Quantum Theatre at the Mattress Factory Garden, 500 Sampsonia Way, North Side. When: Through Aug. 29; 8:30 p.m. Wed.-Sun. Tickets: $15-$25; 412-394-3353.

Down below, a spartan metal cot and old wooden table sit on worn concrete. A fresh-faced young woman confronts a grizzled man. High above, just as the dialogue mentions stars, the clouds part to reveal one star, then many. And at the mention of trains, I swear I heard one of Pittsburgh's distinctive train whistles, far off and ghostly.

The stars and whistles are the kind of bonus you get with Quantum Theatre, which takes the risk that unusual production spaces entail. Weather is a risk, too, but Quantum knows that rain will pass.

The play, "When the World Was Green" by Sam Shepard and Joseph Chaikin, is as simple as its setting amid stone, stars, ivy, past and present. But for all its earthy materials, its 75 minutes do present layers to puzzle out.

The young woman and the old man meet eight times over perhaps several weeks, with intervening monologues. He's in prison for murder; she is supposedly a journalist there to get a story, but we soon learn she has another agenda.

The man is a chef who poisoned his victim in fulfillment of a vendetta going back 200 years. But he killed the wrong man. In a way, it's a mystery: Whom did he kill and why? And why is the young interviewer there? You notice how seldom she takes notes and how passionately the man cares, even though he feigns indifference. So those questions are quickly answered. But gradually the answers seem less sure.

Soon enough you realize plot and characters are secondary to the mythic shape of their quests and to the play's images, which speak more to memory and imagination than to reason and narrative. So responses to "When the World Was Green" will be personal, based on how you respond to oblique art, to poetry and to images of birds or the pull of green water.

It certainly doesn't feel like the archetypal Sam Shepard play about rampant testosterone and measuring up to the American dream. The woman's search is for her father, which oddly parallels the man's life-long search for the object of his vendetta. Does she seek vengeance, too?

She comes bearing fruit -- is she nurturer, avenging angel or even Eve, ambivalent as that image may be? He says his passions have died, but they stir quickly enough.

Designer Tony Ferrieri has shaped the intimate arena to advantage. Lit simply by C. Todd Brown, it boasts crystal-clear acoustics. Seldom does a play seem so immediate and fresh.

Shepard and Chaikin are known for their interest in the actor as real presence, not simply serving a story. So it is of primary interest that Bingo O'Malley and Christine Ryndak present such contrasting acting styles.

O'Malley is brimful of integrity and commitment, taut with emotion. Ryndak is contrastingly pallid and delicate. He emotes; she watches, often silent. His enigma is that of bottled-up passion roiling to the surface; hers is cool and quiet, with just the play of a smile on her lips.

Among the fruit she brings are mangoes. As the play ends, their smell rises on the night air, adding another sensation to this interplay of detail through which we piece together a dreamlike poem of pain and tentative redemption.

There is frustration in how little the play tells us directly. But as director Jason Nodler advises in his program note, don't "puzzle too much about the particulars."

If rain threatens, Quantum will cancel only at 8:30. As of 7:30 you can check at 412-638-7197. A cancellation will be made up on Monday or Tuesday.