EVERY so often, a designer will whip up a dress that purports, temptingly but impossibly, to solve every woman’s figure problems.

There was Diane Von Furstenberg’s wrap, which is great on busty gals but looks like a bathrobe on the flat-chested; Halston’s Infinity, which is impossible on busty gals but brilliant if you’re built like the reedy Pat Cleveland; and Roland Mouret’s Galaxy, which would give even a male basketball centre pneumatic cleavage.

More recently, Stella McCartney’s Octavia has emerged. Also known as the Miracle (celestial names tending to work well here), it’s a simple sleeveless sheath with cunningly located black side panels that optically carve a Jessica Rabbit silhouette, or if you squint the wrong way, a “Scream” costume. Kate Winslet wore it to two movie premieres in (gasp) the same season. Topshop and Bebe knocked it off. I had to try it on.

“Sold out immediately,” said a half-regretful, half-triumphant sales associate at McCartney’s 5,200-square-foot bi-level store, which opened last month, after a banquet suitable for a small wedding, on the aptly named Greene Street (the designer is one of the few in the top tier trumpeting ecological consciousness).

Though not invited, I’d sensed change in the air after a brief tour of the designer’s old meatpacking-district digs the week before Christmas: The place had been deserted, with copies of Gotham magazine sitting desultorily by the entrance and a listless, even rude staff. That location, with its lingering offal stink, had never seemed a good fit anyway for Stella, a devoted vegetarian who eschews animal skins even in her accessories. (One bag line is pointedly called the Falabella, after a rare horse breed.)

But the airy, vivid new shop is an electric prod in the bland flank of SoHo, the onetime arts mecca where it is now not uncommon to encounter anesthetised out-of-towners standing four deep, bumping bags from Victoria’s Secret and Sephora as they gaze into iPhones purchased on the site of a grand former post office.

McCartney is hardly a stranger to mass culture, having collaborated with the Gap and H&M (there’s also the matter of her father, Paul, being an ex-Beatle), but somehow, like him, she has managed to dabble there without losing her essential quirk or likability.

Or at least that’s how I rationalised standing in front of a mirror downstairs with a cavernous Stella McCartney for Adidas khaki weekender slung jauntily over my shoulder. “That is down from $925 to something ridiculous,” said the sales associate, Shawn Gooden, who was shadowing me. (At lunchtime on a springlike Wednesday, we did not have much company.)

“Ooh,” I said, turning this way and that, imagining an impromptu trip to one of the “Downtown Abbey”-like castles where Stella congregates with her friends Gwynnie, Madonna, Liv, et al., or maybe just to the really expensive gym where they whittled themselves down to their prepartum weights.

At its best, however, McCartney’s work has always summed not baronial splendour, nor the grinding rigour of being famous, but a kind of wry whimsy about the puzzle of being female in the 21st century. This she achieves with brawny use of girlish materials (one satin pump, its heel pimpled with raised orange dots, is named the Pelosi — a tribute to Congresswoman Nancy?), pantsuits and tuxedo jackets galore, and repeated appliqués of animals or lush Georgia O’Keeffe-y fruits and flowers.

McCartney is also a mother of four, and since Greene Street stocks her children’s line, I took my three-year-old son there on a Sunday afternoon to buy one of his comrades a birthday present (striped blue tights, $25). Upon entering, he was greeted warmly and presented with a sheet of stickers. After he plummeted down several uncarpeted industrial stairs en route to a bunny-shaped bench (my heart!), Gooden knelt down to cuff his fuzzy pants and made noises about filing an accident report.

“For legal reasons?” I asked cynically.

“No, so we can prove we should make the area safer for kids — buy bumpers, or whatever,” she said.

Whatever accommodations her corporate headquarters approves, Stella McCartney is in no danger of being mistaken for a Chuck E Cheese. The soundtrack is, as one might expect, sophisticated (featuring both She Said She Said by the Beatles and the Rolling Stones’ Mixed Emotions). The appurtenances are graceful, with inlaid blue accents and sculptural hooks in the dressing rooms. And the clothes are divine.

With Octavia unavailable, I tried on her less man-pleasing sister, whom I’ll call Minerva ($1,965): A backless black creation that was like wearing an open camping tent. Then a one-shouldered orange sweater ($595) — baring just one shoulder being very Stella — with a bright, horizontally striped midi-skirt ($1,125): Well-established style no-nos for the lower half, and yet somehow the combination was glorious, better than any generic It dress.