"How about I'm a Castrato … Get Me Out of Here?" Iestyn Davies is trying to think of a name for the reality-TV show he's just devised. It's a rainy January afternoon, and the 32-year-old British countertenor is feeling the effects of last night's bender—as he sips a double espresso in a café in Cambridge, England, it shows in his bloodshot blue eyes. The idea for the series is—well, let's let Davies describe it: "Forget The X Factor—let's see the castrato! Everyone would want to know whether he could have sex. Nothing much has changed: The opera audience in the 18th century were exactly the types who'd be buying gossip magazines today to find out who's shagging who." Davies perks up as he concludes the pitch: "There must be someone out there thinking, 'I'm so desperate for my child to be famous and make money that I'll put them forward for castration!'"

As an elite countertenor, Davies is accustomed to being described as a modern castrato—and laughing it off. It's true that with their rare ability to either sing at the absolute top of the tenor range or sail ethereally into falsetto, countertenors now assume the roles castrati once performed in Baroque operas. But that's where the similarities end: In order to preserve their lush treble vocals, castrati were given the most unkindest cut of all in a warm milk bath—if they were lucky, they also got some opium—before they reached puberty. Thus depleted of testosterone, they grew up with delicate, feminine features and—if they were unlucky—an enormous barrel chest, weedy pin legs, and a shrunken penis. That's not Davies, who, in profile, bears a striking resemblance to Daniel Craig and, needless to say, remains fully intact. His voice is the sole result of exhaustive, almost maniacal training. "I've found jogging and swimming are great for the respiratory system," Davies says. "I usually have three to six hours of rehearsal the day of each concert—it's like the day of an Olympic event."

Castrati began to go out of fashion in the late 18th century, and in 1878, after Pope Leo XIII closed the Sistine Choir to aspiring castrati, they effectively vanished. Now, after a couple of centuries in the shadows, men who can hit a high C are again finding an appreciative, even obsessive audience. Countertenors like David Daniels and Andreas Scholl have been slowly building a cult following over the past decade, but Davies—handsome, young, charismatic—could be the one who finally proves that these singers are more than a curiosity. After performing in nearly every major opera house in Europe, Davies made his debut at the Metropolitan Opera in New York last December, then sang at Carnegie Hall, prompting the New York Times to marvel at his "potential to be one of the truly special artists of his generation." Female admirers aren't quite screaming, "Long live the knife, the blessed knife!" as they did in the 18th century, but Davies' growing fame does present some unusual quandaries. For instance, what do you do when the queen requests an audience but opening night of your biggest role, at the English National Opera, is coming up? If you're Davies, you don't pass up the chance to rub elbows and drink champagne with the royals. A few days before the performance, he came down with pharyngitis (not necessarily connected, notes Davies). Left without his voice, he had to mime his part while someone covered his singing offstage. "Oops," Davies says.

We're sheltering from the rain and caffeinating as Davies kills time before a rehearsal session for an album of verse anthems by Henry Purcell and Pelham Humfrey that he'll be recording with the Choir of St. John's College. Davies, a native of York, has a long history here, having arrived at St. John's (whose alumni roll includes British prime ministers, Nobel Prize winners, and three saints) as a boarding-school student on the eve of his eighth birthday. That sense of homecoming is the reason he was celebrating last night, he explains—normally he treats his vocal cords with a reverence verging on zealotry.

Leaving the café, we wander past the university's immaculately manicured lawns and into St. John's chapel. Davies sang here almost every day between the ages of 8 and 13, when he held the prestigious position of head chorister. But then, just as he was graduating, Davies' voice broke. "It was traumatic," he says as we look down the nave. Davies continued to sing, but he knew that he was nothing special as a bass or a tenor: "After your voice breaks, you lose the ability to do the thing that made you stand out."

In high school, Davies formed a pop band, Cage. He saw an ad in the back of a music magazine—"Are you between 16 and 21? Do you want to be famous?"—and sent in a demo tape and a photograph of the group. On the strength of the package, the A&R team at Epic Records predicted that one track, the Britpop-inspired anthem "Up and Above," had the makings of a No. 1 hit. But Mrs. Davies had other ideas. "My mum didn't want me to do it because she was worried I would blow up my future," he says with a chuckle. "Which is probably true."

Not long after the Epic deal fizzled out, Davies was messing around with chums still in the school choir when, on a lark, he started singing falsetto. To Davies' surprise, they didn't laugh. They just listened in wonder. "I think, cynically, that I liked it because it made me stand out again," he says. "It's a cliché, but I'd literally found my voice again."

One week before we meet in Cambridge, Davies is standing center stage in the Liverpool Philharmonic Hall, waiting patiently for his cue. It's the afternoon rehearsal before tonight's big performance of Handel's Messiah, which will mark the 47th time that Davies has performed the oratorio in a career that is not yet a decade long. From the moment he opens his mouth to sing, the 30-strong orchestra and 100-odd members of the chorus are riveted. There's a feeling in the air that we're about to hear something, to use a word Davies dislikes, otherworldly.

Davies' voice is about to ring out across America. Beginning at the end of February, he'll take the stage at Chicago's Lyric Opera—where he'll be performing alongside David Daniels in Handel's Rinaldo—and in October, he'll make his triumphant return to the Met in New York. A few years ago, the idea that a countertenor could headline a big show at the Lyric Opera (much less the fact that the Met would commit to booking Davies for two more seasons) would have been unimaginable. "Audiences in the States are so ingrained in big Italian operas," Davies says. "Maybe people think, 'I don't want to hear vegetarian opera—I want to hear full-blooded singing.'"