WHEN A PLAY is billed as "challenging," it usually means the subject matter has been deemed provocative to delicate theater-going sensibilities. Most likely, you're in for actors yelling politically incorrect things at each other, or being secretly racist. Someone will probably say the c-word.

It's rare to see a script that's challenging on its literary merits. (defunkt theatre used to be a reliable source for this sort of thing, but changes in their artistic direction have taken them in a more accessible direction in the past few years.) But that's exactly what Naomi Wallace's One Flea Spare is: a tough, surprising show that takes some work to unpack.

In plague-ridden London in the 17th century, an elderly rich couple (venerable locals David Bodin and Jacklyn Maddux), a sailor (Matthew Kerrigan, excellent), and a girl (Kayla Lian) are quarantined in an estate, forced by proximity to navigate shifting alliances of sex and class. Lest we take this setup too literally, much of the show's music is contemporary, and pointed in its politics (Rage Against the Machine's "Killing in the Name," KRS-One's "Sound of da Police"). There's only one conclusion: This is a show about class, and it's relevant now.

Shaking the Tree's production neatly balances high-concept, open-ended allegory with grounded character interactions. The show mines the humor in the script without allowing jokes to overpower moments of genuine discomfort, of which there are several. (There's some weird sex stuff, and some weird fruit stuff.) The performances, too, are lived-in and committed. This isn't an easy show, but Shaking the Tree sticks the landing with confidence and grace.