Imagine walking down Fifth Avenue on a Saturday afternoon, and….oops, your knickers fall to the cement.

The horror.

Yet that’s the crisis at the heart of “The Underpants,” a quirky off-Broadway show about a man whose wife can’t keep her drawers on. The show adapted from German by Steve Martin, opens tonight at the Classic Stage Company.

God forbid you’re struck by this mysterious (and fortunately rare) condition, remember mom’s advice: Make sure your underwear is clean. More important, make sure it’s fashionable.

To accomplish this, check out our guide tot he season’s best undie styles and the stars who’d look great in them.

Girly panties

Undies: Araks’ butterfly beauties

Who might wear them: “Girly-girl” Jennifer Love Hewitt

When to wear: For a walk in Central Park with your spring fling, wearing a cute sundress or a light chiffon skirt with a loose, flower-print top.

Where to find them: $55. Call 1-800-576-7232 for stores in your area.

*

Tomboy-friendly tap pants

Undies: So Low’s pink jersey briefs

Who might wear them: Cameron Diaz

When to wear: When you’re feeling frisky, around the house. Think Cameron Diaz dancing in her Underoos in “Charlie’s Angels.” Best worn with a “beater-T.”

Where to get them: $26. http://www.girlshop.com. Call 1-888-450-SHOP for stores in your area.

*

Sentimental skivvies

Undies: Victoria’s Secret’s string bikini

Who might wear them: Sarah Michelle Geller

When to wear: Cuddling with your sweetie. Victoria’s Secret can customize their classic second skin satin panties with the name of your beloved in Swarovski crystals.

Where to buy: $14 (crystals cost $1 a letter). Victoria’s Secret stores. Log onto http://www.victoriassecret.com.

*

Funky bloomers

Undies: La Perla’s red hot, g-string attached to skirt combination

Who might wear them: Dare-devil diva Lil’ Kim

When to wear: Good bedroom lingerie that literally might make your man fall off the bed, but if you wanted to wear it out for a night and surprise him later, a knee-length black cotton skirt could keep it hidden well.

Where to find them: $141 at La Perla, 777 Madison Ave., (between 66th and 67th Sts.) Call 1-888-LA-PERLA for more information.(Insider tip: La Perla’s sister collections Marvel and Simply have equally sexy undies for under $100.)

Ethnic print underpants

Undies: Tahiti boyleg from Cosabella

Who might wear them: Sexy Latina star Shakira

When to wear: You could wear these to the beach with a lavender wrap tied around the waist. Or give a little sneaky peak of the palm trees with a pair of old Levi’s and a sexy halter top.

Where to find them: $50. http://www.estilonet.com. Or call 1-866-ESTILONET.

Campy boxers

Undies: Loose, lacy undershorts from The GAP

Who might wear them: Virgin teen Mandy Moore

When to wear: Taking the dog for a walk around the block (for the girl willing to take a risk), with a comfy T-shirt. Or working on the great American novel at your computer station on a Sunday.

Where to find them: $19.99 at area Gaps. Call 1-800-GAP-STYLE for stores in your area.

Fierce knickers

Undies: Sailor Jerry’s tiger bikini set

Who might wear them: Animalistic actress Angelina Jolie

How to wear: For a night out on the town with Prince Charming. Wear them under hot leather pants with the top to match or a simple, lace camisole.

Where to find them: $32. http://www.girlshop.com. Call 1-888-450-SHOP for stores in your area.