Every now and then, a grandma will tell you how great it is to have grandkids – they come, they share the love, and after a while their parents take them away. But what happens if they return way later, without anyone to take them back home?

That's a nutshell description of the plot of Amy Herzog's lovely "4000 Miles," an adroit piece of playwriting -- funny, sad, moving and altogether effective in a Philadelphia Theatre Company production that opened Wednesday night. The play was first produced Off Broadway two years back, then again last year by Lincoln Center Theater, where it was a hit. The Philadelphia Theatre Company's production is among the first regional productions of a play that will probably have dozens of them.

And that's not because this story, of a young man who starts on a biking trip in the state of Washington and ends up unannounced at his grandmother's Manhattan apartment, caters to any particular audience or taste. It's because "4000 Miles" is such finely tuned playwriting. Herzog tells us what's happening in this grandson's life by planting little revelations, one by one, in just the right contexts all the way through.

There's a clear underlying tension in the play. What does this guy want from his grandmom? What does she get from him? "4000 Miles" is a story about two generations separated by a missing one – the man's parents – that looms large in the conversation but not in this newfound relationship. In a subtle and organic way, it becomes a story about a beneficent and short-lived co-dependence.

Organic is a good word for these two characters: The way Herzog draws them, each is colored in several shades of gray, making both feel very real. (The playwright says her own grandmother lives in the dialogue.) The somewhat impetuous, impulsive grandson is at home with a mode of street-talk he would use with his 20-ish friends. His grandmother, a lefty from way back who's seen it all, is at home telling him just what's on her mind.

And so a complex relationship builds, one of extremes in age but on a common ground. That relationship, in the Philadelphia Theatre Company production, has a surprisingly earthy chemistry in the performances of Beth Dixon as the grandmother and Davy Raphaely as her grandson – each holds the stage with a singular presence that embodies the characters.

Dixon's grandmother may be losing her words and also her keys, but never her sense of candor. When the actress tells her grandson her response to a husband's paramour – "There are people you like and there are people you don't like, and I don't like you" – you sense in her delivery not just a fighter, but a refreshingly practical person. Dixon's performance seamlessly blends age with wisdom; she dotters around in the style of someone losing her balance, and talks all the while like a woman whose world remains in sharp focus. She acts wonderfully with her eyes.

Raphaely, a Philadelphian who has been acting on stages here since his teens (most recently as Arden Theatre Company's "Pinocchio" in a family production) , climbs to a new level as the grandson. He gracefully shifts moods when called for, and sometimes his posture speaks for his character as well as the words he's delivering. If Dixon is the finely nuanced codger, Raphaely is the ever-maturing fledgling. Shannon Marie Sullivan and Leigha Kato round out the cast in two smooth portrayals – one a girlfriend, the other, a possible squeeze.

"4000 Miles" marks the welcome return of Mary B. Robinson to a Philadelphia stage – if you're a veteran theatergoer here, you'll remember her as the artistic director of the long-departed Drama Guild. Robinson's direction of the play brings out the fluid sense of Herzog's story, and the interpretations she gets from Dixon and Raphaely mine the richness of the two characters. She's greatly aided in bringing "4000 Miles" to life by Jason Simms' handsome apartment set and Thom Weaver's lighting design, which has the audacity – and the smarts – to sometimes cast the characters in near darkness.


"4000 Miles" runs through Nov. 10 in a production by the Philadelphia Theatre Company at the Suzanne Roberts Theatre, Broad and Lombard Streets. 215-985-0420 or www.philadelphiatheatrecompany.org.