Here's the story from Slate:
[Martha Plimpton] has a budding career as a singer—and who would have thought that this punky misfit actress would turn out to be a fabulous cabaret diva? Last night, I caught her at Joe's Pub in New York's Public Theater with Julian Fleisher in a show called Save it for the Stage. (It was one night only, but they'll pop up again.) Plimton and Fleisher compare themselves to "Steve and Edie, Sonny and Cher, Bonnie and Clyde, and Leopold and Loeb," which should give you some idea of the (unrehearsed) onstage banter. But if the act borders on camp, Plimton sings with her whole heart: She has a chesty but soaring voice, and with her short blond hair and slinky body she looks great when she's contorting herself in front of a microphone.
Her rendition of "Neverland" was too earnestly plaintive, but everything else was a joy: the opening medley of "Movin' On Up" (from The Jeffersons) and "Nine to Five; "Little Red Corvette"—'70s/early '80s songs revitalized by her stylings and the witty band. Plimton will bring down the house with a number, then break character and shrug and squeal as if to say, "Was that me? Did I just pull that off?" It's so exhilarating when you discover that an actor whom you loved (and I've followed Plimton since The River Rat in 1984) has pipes.
I'm not sure how I feel about this. If she's serious about the act, it could be great. If she's being ironic, though, I'd be terribly disappointed. Of course, I'd still love her in my high-school heart.