“Civilized and delightful, but not very lively”—so sniffed the local daily back in the day, when Arena Stage gave D.C. its first look at Tom Stoppard’s Arcadia. The local alt-weekly disagreed, and still does: The play’s full of comic potential, and Aaron Posner’s keen, lucid production goes well beyond lively to flirt with the antic. And the result, we’re delighted (in our civilized way) to report, is something very much like bliss. Now it’s true, Arcadia can seem daunting if you worry too much about all those arcane arguments involving the Romantic poets and the higher maths. The fact is, though, that it’s basically a sex comedy with mournful metaphysical inclinations, a history-mystery play larded with smart structural games inspired by the questions that obsess its characters. But if it’s always been bracingly intelligent, it’s never been quite so much fun; Doug Wager’s 1996 staging at Arena leaned on the story’s wistful notes, but Posner plays up the impishness and the joy, casting actors whose timing is as crisp as their diction, and whose eyes seem always alive with both curiosity and mischief. (He’s cast it exquisitely, too—a dozen players, all splendid, so if you don’t see their names here it’s no slight.) The intertwined storylines—landed gentry talk landscape gardening and Newtonian principles at a splendid Derbyshire house circa 1809, while academics dig through the family’s library and its secrets in the present day—is anchored by pitch-perfect turns from Erin Weaver and Cody Nickell (as adolescent math genius Thomasina and her witty-rascal tutor Septimus, the central 19th-century twosome) and Holly Twyford and Eric Hissom as Hannah and Bernard, two modern-era researchers whose mutual suspicion never quite poisons their shared love of the chase. And so while the aches of Arcadia do linger—not for nothing is its title a name-check on that famous acknowledgment of how we all end—its pleasures seem richer than ever. In a time of gloom and caution, in the theater and in the world beyond it, they are welcome indeed.