It is the wisp of a dress that in 2002 gave the fairy-tale romance between a British commoner and a blond-haired young prince a racy—and wholly modern—kick start.

On March 17, the leave-nothing-to-the-imagination covering that Kate Middleton wore during a charity fashion show while a student at the University of St. Andrews—and that transformed Prince William’s interest in his classmate from platonic to romantic—will be sold to the highest bidder by Kerry Taylor Auctions in London.

The hey-baby dress, by former student Charlotte Todd, will be sold alongside hundreds of far grander garments, many of which tell the history of Western design. For example, a 1920s beaded evening cape by Gabrielle “Coco” Chanel was once owned by the eccentric Italian fashion editor Anna Piaggi, and a rare silk Mondrian dress by Yves Saint Laurent comes from the collection of American collector Sandy Schreier.

Yet the only frock that matters—sorry, fashion purists—is lot number 293, the very last in the sale. Todd’s black-and-gold scrim of hand-knit silk, edged in turquoise elastic, was inspired by the charity show’s theme “the art of seduction.” Designed as a skirt, it was jury-rigged into a dress by Middleton—or by some stage-hand with an eye for provocation. The resulting look gave the entire audience, including Prince William, a clear view of Middleton’s fine personal assets. Lest anyone judge Middleton’s fashion brazenness as inappropriate or an example of the decline in proper comportment, the auction also includes several boudoir ensembles worn by the Duchess of Windsor. Her intricately embroidered nightgowns—one in bright fuchsia—have more fashion panache, but they are titillating in their own way.

What distinguishes Middleton’s dress (and that term is being used loosely here) is its lack of seductive finesse. It’s a look-at-me, sexual shout-out, reflecting both the youth of the wearer—Middleton is now 29—and the culture of its times. It resembles the sort of going-out garb young women weaned on Sex and the City might choose. It recalls Carrie Bradshaw’s blurring of the lines between high fashion, low style, and kooky costume: a cultural moment when bra straps were accessories, tutus became skirts, and a pair of knit knickers were perfectly acceptable “pants” for a night trawling the clubs.

Indeed, there’s something endearing about the emphatic and awkward sexuality of the skirt-turned-dress. What woman, now the embodiment of self-possession, can’t look back on some mildly scandalous frock whose sole purpose was to get the guy?

Gallery: Kate Middleton Through The Years

And what young man, royal or common, would have had the willpower to simply look away? Todd, who has been working in the gift shop at a Bristol aquarium, is first to note that the skimpy tube of lace offers little in the way of elegant design. “My interest was in textiles,” she told me. “But we were doing a fashion show, and so I needed to make it into a garment.”

“I had an idea the relationship would go the distance,” Todd, 31, told me. She had tucked the fashion project away with the rest of her college mementos, but at a certain point keeping the dress ceased to be an option. “There was so much interest from overseas—it was quite a lot of responsibility on my shoulders.”

Taylor expects the dress to sell for anywhere from £8,000 to £10,000—or about $11,000 to $14,000. Proving that in this case, provenance—and prescience—can be quite lucrative.

But even being the palace’s de facto modern matchmaker doesn’t guarantee an invitation to the royal wedding. Todd, a newlywed herself, said she expects to be watching it on television, just like everyone else.

Robin Givhan is a special correspondent for style and culture for Newsweek and The Daily Beast. In 1995 she became the fashion editor of The Washington Post where she covered the news, trends and business of the international fashion industry. She contributed to Runway Madness, No Sweat: Fashion, Free Trade and the Rights of Garment Workers , and Thirty Ways of Looking at Hillary: Reflections by Women Writers . She is the author, along with The Washington Post photo staff, of Michelle: Her First Year as First Lady . In 2006, she won the Pulitzer Prize in criticism for her fashion coverage. She lives and works in Washington, DC.