'We put the human condition onstage and make it dance." True to the narrator's word, Pig Iron does just that in

Chekhov Lizardbrain

, the theater company's most successful brainchild. The show, under Dan Rothenberg's direction, briefly returns to Philadelphia, where it began, trailing clouds of glory from its New York run.

This elusive, endearing play focuses on Dmitri (James Sugg in a mysterious and subtle performance), a botanist from Portland, Ore., who returns to Oswego, N.Y., to visit with three childhood friends whose mother has recently died.

Sascha (Chad Lindsey) has been living in the old house, while his two older brothers, Nikolai (Dito van Reigersberg) and Pyotr (Quinn Bauriedel), are cruelly but sensibly eager to sell it. Thus, this play, like so many Chekhov plays, is about real estate.

Sometimes they appear in normal contemporary clothing, but sometimes they wear long white underwear with top hats and fake mustaches. In the circular playing space, with its red ropes and red curtain, it all suggests a bizarre circuslike atmosphere - not a bad setting for the inside of a brain. And when Chekhov Lizardbrain, Dmitri's alter ego, reveals that under his hat is a tiny house, it becomes a trigger for a brief discourse on memory ("the first rule of consciousness: Memory is not like film"). The distinction between "the idea of a thing and a thing itself" (thank you, Wallace Stevens) is crucial to sense perception, to memory, and, of course, to theater, where what we see is never, really, real, the thing itself.

Chekhov Lizardbrain is also about theories of the brain ("You want to know something? You gotta have something to know it with"). The reptilian brain governs the most basic level of functioning - breathing - with evolutionary layers adding functions beyond that: emotion and then language. Sugg sometimes appears as Chekhov the ringmaster, in top hat, sometimes as an invisible presence who speaks in another voice. Whether what we're watching is a study in schizophrenia or in autism (flat affect, stiff fingers, need for rules, numbered lists) hardly matters. "The menagerie of human possibilities" is rich and full.

The earlier version of this show, revised apparently to make it more accessible, put more focus on the characters (the brothers' personalities previously seemed more distinct and thus the Three Sisters connection more tantalizing). But the new self-referential jokes - worrying whether the audience is growing restive or whether a crucial prop has broken - are so odd and so charming.

The play, written by Robert Quillen Camp in collaboration with the original company, gives us much for each segment of our brains to work on (beyond the necessary saurian breathing): Emotionally and intellectually, there is plenty to keep you busy musing.