Kevin Plank likes to tell stories. Likes to so much that one of the four credos of his multimillion- dollar company is "Tell a GREAT story."

His favorite? The one that comes closest to telling you all you need to know about Plank, about Plank's company and about the sports business revolution that's taking place in a revamped warehouse on the Baltimore waterfront? Like most of Plank's stories, it starts with an athlete, Jeff Conine of the Florida Marlins. It's three hours before a 2003 NLCS game at Wrigley, and Conine is cold. He leaves the clubhouse, walks five blocks to a sporting goods store and throws down $50 of his own hard-earned for a long-sleeve undershirt. Walks back to the clubhouse, and because he knows displaying unlicensed logos is against MLB rules, has the equipment manager blot out the unapproved mark. Takes the field. The hidden logo: Under Armour.

Under Armour is hot, and cool, and any other weather-related adjective connoting consumer-culture hipness. So wet, in fact, that pro athletes not only pay to wear the products but have endorsed them without demanding compensation. It's hip in a grassroots, I-was-there-at-the-beginning way, with the devotees-a huge number of them youth and high school athletes-treating their gear like the garage-band recordings and underground mix tapes of their favorite acts. The brand's ascendance from niche company to pop-culture symbol is the type of legend that hasn't hit the sporting-goods scene since a guy named Phil Knight and a company called Nike started making goofy-looking, waffle-soled running shoes in Eugene, Ore., in 1974. Just like Nike, and just as the legendary Knight has announced he is stepping down as CEO, Under Armour seems ready, willing and able to influence young minds through product, advertising (their ads appear in this magazine, as well as several others) and, of course, stories.

Plank, the 32-year-old founder and president, describes second-graders, unprompted, chanting his company's over-the-top motto ("We must protect this house!") during a recess dodge-ball game at a Maryland elementary school. He talks about taking calls from Roger Clemens, who likes Under Armour so much that he personally places orders to outfit his four sons' sports teams. The company's electronic sales pitch to retailers includes footage of Letterman screaming that he will, indeed, protect his house on Late Night, and Oprah reciting the advertising slogan, a bit more calmly, in the afternoon.

A Notre Dame chaplain requested a copy of the "Protect This House" ad so he could write a pregame speech based on the words spoken by Maryland's Ralph Friedgen, who plays the coach of Under Armour's fictional football team. A newlywed in South Carolina sent a video e-mail to UA's marketing department, depicting him and his groomsmen surrounding his bride at the reception, reenacting a scene from the commercial, and telling the young lady they will protect her house. (Whether it was meant figuratively or literally, the groomsmen didn't say. Probably depends on the neighborhood.) Pro athletes routinely call Marcus Stephens, Under Armour's creative director, asking to be in the next commercial.

All of this fuss over expensive underwear?

As the story goes, Plank was a special-teams captain at Maryland in the early 1990s. Disgusted with having to change out of a sweat-soaked, heavy cotton T-shirt two or three times a practice, Plank figured there had to be a better way. After he finished playing in 1995, he set out with a modest goal: create a lighter, tighter, drier shirt to wear under football pads, something approximating the fit and feel of the bicycle-style shorts he wore under his pants. He went armed with $40,000 in credit and $20,000 in cash from two previous business ventures: selling T-shirts at concerts in high school and selling roses at Maryland.

He drove his Ford Explorer to New York's garment district, where he told suppliers and manufacturers his dream of making a synthetic shirt that would wick moisture, fit snug against the body and weigh next to nothing. "You have a pattern?" one asked, to which Plank replied, "No, what's a pattern?" Some were patient with the wide-eyed 23-year-old. Some were not. But eventually, Plank left New York with a prototype.

An artistic friend designed the logo, and the name ... well, Plank likes this tale, too. He planned to call the company Body Armour, until a friend at the patent and trademark office ran the name and uncovered too many obstacles, including, predictably, a New Jersey auto-body shop. This was a setback. Body Armour seemed to be the only way to describe his product. Depressed and out of options, he arrived at his brother Bill's office one day to pick him up for lunch. Filling the vital role of obnoxious elder sibling, Bill struck Kevin with a sarcastic jab: "How's that company of yours doing? What do you call it, Under Armour?"

Comic-strip lightbulbs went off in Kevin's head. He looked at Bill and said, "Under Armour! you know what? I can't go to lunch. I've got something to do." He went back to his car, drove to the patent and trademark office and filed the paperwork. Just another funky, unplanned, profitable step in the annals of modern business.

Plank's business life is heavily influenced by his sports career. Drawing on his experience in locker rooms, he targeted college equipment managers instead of athletic directors and university presidents. He asked the Maryland equipment manager if he had authority to buy undershirts; he discovered that many Division I-A equipment managers have a budget in the neighborhood of $300,000 to buy everything from helmets to socks.

Packing his light polyester and Lycra prototypes into his Explorer, he drove south, through the heat-drenched ACC. Along the way, he employed a little psychology. He knew that equipment managers spend their days being ordered around and harassed, when they're not being outright abused. He marketed to them, speaking their language and giving them the respect they don't always expect. (Under Armour still throws a lavish party for equipment managers at their annual convention.) Tom Conner of Georgia Tech made the first significant Under Armour purchase, in 1996, somewhere between $7,000 and $8,000 worth, and Plank was on his way.

Conner loved the product-it cut down drying time, for one-but he assumed he was buying from an established company looking to expand its reach. A year later, he met Plank to place another order. "Thanks, Tom," Plank said. "You were the first guy to believe in me."

Conner started laughing. "You're lying."

Plank was dead serious. "No, I'm not."

Now, says Conner, "Look at them; they're an empire."

Plank's sales pitch overcame a slight misgiving. "We were trying to convince these football players to wear glorified lingerie," he says, chuckling. "That's the irony-getting these big, tough football players to wear women's lingerie on their upper body. It might have had a masculine cut and a bold logo, but for the most part it was still lingerie." Yet he's found that men are flexible when it comes to fabric. "It just feels so good on the body," says A's pitcher Barry Zito, an Under Armour devotee and occasional pitchman.

From the beginning, Plank called in his personal connections, not only from Maryland but also from Fork Union (Va.) Military Academy, where he spent a year playing football and learning not to be a knucklehead (his word) after high school. The inspiration for Under Armour's first motto, the short-lived and long-forgotten "Tite but Nice," was a former player at Maryland, an offensive tackle named O'Neil Glenn who would admire himself in front of a mirror before games and finally proclaim, "I feel tight-but nice."

Thirteen of Plank's teammates from Fork Union, including Eddie George, played or are playing in the NFL. Plank never hit up his ex-teammates for money, but he asked a simple favor: wear this shirt, and if you like it, tell the guys in the lockers next to you. Repeat as necessary.

This type of grassroots networking, called "viral marketing" in the business, is a hallowed subject in the Under Armour corporate culture. One of the first players to spread the word was former Maryland and current Dallas defensive end Eric Ogbogu, "the Under Armour guy" who leads the fictional football team's tribal sloganeering.

Ogbogu (pronounced Uh-BAH-goo) is far better known for this role than for his eight-year NFL career. As he walks off the practice field, Cowboy defensive coordinator Mike Zimmer often tells him, "Let me hear it." That's Ogbogu's cue to let loose with the slogan. Ogbogu went to see a friend's son play high school football recently, and arrived just in time to see the team gather on the sideline and do their own rendition of "Protect This House." True-to-life mannequins of Ogbogu's 6'4", 269-pound body, made at a Hollywood body-cast shop, are everywhere in the Under Armour offices. They call these muscled forms the Big E, and the hulks lurk around every corner, like statues in a church.

Under Armour uses LaVar Arrington, Mark Prior and Zito as endorsers, but there are no high- profile, Jordanesque pitchmen who could overshadow the product. "How realistic is it if you're paying someone to feel a certain way?" Plank asks.

Professional skier and former Colorado football player Jeremy Bloom was one of Under Armour's first athletes, using the product as a pro freestyler before he entered college. Other athletes have plugged the stuff in exchange for a donation to charity. "Athletes have endorsed Under Armour for nothing," Bloom says. "It's like you're in the family if you wear it. You're in on something."

Any parent of an athletic kid knows the powerful lure of the brand. It might be lost on Mom or Dad, but Junior's in on it. Watch a high school football game, and it seems half the players on the field are wearing at least $100 worth of UA gear-socks, undershirt, gloves, skull cap. The youth-market emphasis gets back to personal experience for Plank, the youngest of five boys. "I know there's always a little kid out there who wants to look like his big brother."

Need numerical evidence of Under Armour's influence? The company began selling football gloves this year, and claims it immediately captured 23% of the market. Sales for all football gloves, regardless of brand, shot up more than 30% after UA introduced its line.

Nike and Reebok have developed lines of performance clothing to compete with UA's "compression wear," but Under Armour's 70%-plus market share would make Microsoft proud. Georgia Tech is still wearing cold-weather shirts it bought eight years ago.

The brand's popularity wreaks havoc with some schools' "exclusive" supplier deals. "When players spend their own money they buy Under Armour," says one D1-A equipment manager. UA salespeople return to Baltimore with stories of travels to non-Under Armour universities, where they often hear players tell equipment managers, "Get me one of those Nike Under Armour shirts."

In jeans and a company sweatshirt, Plank's frat-boy looks are hardened by eyes that narrow once he becomes engaged in a subject. "He's dedicated, and he always had something working," Ogbogu says. "Even in college, he could sell you anything." Plank acknowledges his improbable rise, but he's over the look-how-young-he-is phase of success. What few people understand is the work it took to create the apparent lightning-quick fortune.

The company's first headquarters was a Georgetown row house left to Plank by his grandmother. He stored and shipped out of the basement, and jokes that he would answer the phone with an authoritative "Under Armour." When someone would ask for Kevin Plank he'd say, "Hold on, let me see if he's free."

He enlisted the help of former Maryland All-America lacrosse player Kip Fulks, now an Under Armour partner, to keep up with the orders. One day he saw Fulks licking a No. 10 envelope and slapping on a stamp. Fulks said experimentation had led him to discover that a shirt, rolled just so, could fit in a business-size envelope, thereby saving on shipping costs. Plank gently told his friend he didn't think customers wanted to receive premium clothing delivered that way.

This was back before he hit it big, before he created another break by getting his clothing in Oliver Stone's Any Given Sunday, back when he was idealistically telling anyone who'd listen that every kid in America would eventually wear his T-shirt under football pads. "Even back in Grandma's basement," Plank says, "I never thought it couldn't happen." If you can follow the bouncing logic, Plank's next statement-"I was just smart enough to be dumb enough not to realize what wasn't possible"-makes nearly perfect sense.

The popularity comes despite pricing that seems, well, steep. Long-sleeve ColdGear shirts run as high as $60, but Plank unapologetically says he's producing a premium product. The fact that Under Armour will exceed $200 million in revenue is testament to America's trading-up philosophy, which has made the world safe for $5 mochas and $100,000 Hummers. But at a sporting goods store in Portland, Plank watched a single mother with two young boys stand in front of an Under Armour rack and get pestered by her older son. He really wanted a shirt, but she recoiled when she saw the $30 tag. "That killed me," Plank says. "I want her to be able to buy that shirt." Next year, UA plans to offer a performance shirt in the $20 range.

There's a bouncy corporate culture at UA headquarters, with cheery employees talking of "brand authenticity" and teamwork with cultish zeal. From Plank down, the movers and shakers are young, ridiculously so for an established company. The median age, it seems, is roughly 12. Raphael Peck, UA's vice president of apparel, cites a hypothetical example of the corporate philosophy. "Under Armour would never run an ad with a single skinny person running down the street with the line, 'There is no finish line.'" What would Under Armour do? Peck grins as he envisions the scene in his head. "Us? We'd show a group of 220-pound guys on a training run with the words, 'Lions hunt in packs.' "

Makes sense. Lions, after all, are cool. But there's nothing cool or hot or trendy about a lonely runner. There's no story there. Besides, loneliness raises an existential question: is an empty house even worth protecting?