The best dark comedies start small - especially comedies about family. Beginning simply, they first establish the broken dynamic between just two or three characters, before gradually layering in the rest of the bickering, disjointed extended family.

Then, in most cases, an outsider slips in, someone new to the tribe, and suddenly ... everything gets worse.

Yes, occasionally, the opposite happens, the stranger actually makes things better, reminding everyone how much they really love each other, bringing them all miraculously together again.

But where’s the fun in that?

In the play "Becky Shaw," directed with wicked glee by Barry Martin and playing one more weekend at 6th Street Playhouse before moving to the Napa Valley Playhouse for two more, playwright Gina Gionfriddo takes both of those approaches, the destructive stranger and the savior stranger, and mashes them all together into one disturbingly riveting, weirdly funny story unlike any other play I’ve seen, all while adding some of the most scathingly hilarious dialogue in recent memory.

Suzanna is an aimless college student wrestling with existential angst.

Her late businessman father apparently squandered the family fortune, her mother Susan has taken up with a much younger man, and she’s just had a one-night stand with her adopted brother Max.

Things are, to say the least, complicated.

Urged by Max to take some time to find herself, she impulsively meets and marries the amiable Andrew, a good guy with a need to save people and fix them, clearly attracted by the fixer-upper that is Suzanna’s life.

Andrew works with a fragile but somewhat interesting woman named Becky Shaw, whom he thinks might be a good match for Max, and when she arrives for the blind date, awkwardly overdressed in a fluffy pink dress Max derides as looking like a birthday cake, it is clear that things are about to go seriously south.

For one thing, Max is feeling territorial about sharing his sister-lover with Andrew.

For another thing, something is clearly very wrong with Becky Shaw. Whether she is emotionally unhinged or merely pretending in order to wheedle her way into this not-so-stable family is the question the play slowly, gradually explores.

There are two ways to play a character like Becky.

Some would make her as normal as possible, at first, and then gradually drop little hints as to exactly how weird - and potentially dangerous - she might be. Others will let you know from the first entrance that we are looking at a mad woman. That’s the approach actress Taylor Bartolucci takes, and she runs with it, giving Becky a wildly entertaining series of ticks and jitters that will delight some while possible making others wish she’d taken the more subtle, gradually unfurling approach.

That is just one of many juicy topics audiences will want to debate after catching "Becky Shaw," a play packed with ideas, written with a brilliantly spot-on understanding of how families work, and sometimes stop working. The actors have a great time with this material, especially Bartolluci and moillie boice, who as Susan, the matriarch, gives another in her series of magnificently ballbusting performances.

Whatever you end up deciding about Becky Shaw the person, it will be difficult to deny that "Becky Shaw" the play - among the best new dark comedies to come along in recent years - is deeply challenging, charmingly infuriating, and painfully hilarious.

"Becky Shaw" runs Thursday–Sunday through July 13 at 6th Street Playhouse, 6thstreetplayhouse,com, then runs July 18 through 27 at Napa Valley Playhouse, napavalleyplayhouse.org.