WHEN SOMMELIERS toss around words like “besotted” and “Holy Grail” to describe a wine, I take notice. When that wine turns out to be a Cabernet Franc from France’s Loire Valley and not some famous Bordeaux or grand cru Burgundy, I’m intrigued. And when it can’t be found on restaurant wine lists or retail store shelves, I get on a plane to check it out.

That’s how I came to visit Clos Rougeard, in the Saumur region of the Loire Valley, last month. Beloved by industry insiders and wine geeks the world over, the estate’s wines are the sort that sommeliers approach “on their knees,” a reverential professional told me.

Clos Rougeard is, in many ways, an unlikely candidate for a cult following. First, the Loire Valley has produced few expensive or sought-after wines compared with Bordeaux, Burgundy or even the Rhône Valley. Second, Clos Rougeard specializes in Cabernet Franc, a grape that many drinkers actively dislike, especially when it’s grown in the Loire, where it can take on a weedy, herbaceous note.

Founded in the late 17th century, the winery has been in the same family for eight generations. Brothers Nady and Charly Foucault took over in 1969 and ran the estate together until Charly’s death last December. Well-respected locally, the Foucaults were mostly marginal figures in the world at large until fairly recently, according to Jim Budd, a London-based Loire Valley authority and author of Jim’s Loire blog.

The 2005 vintage was a turning point for the estate, according to Jake Halper, portfolio manager at New York-based David Bowler Wine. U.S. buyers worked themselves into a frenzy trying to get their hands on those bottles, Mr. Halper recalled. “Prior to that, Clos Rougeard was out there,” he said, meaning it was readily available. “But people started talking about 2005 like it was a legendary vintage like 1947 in Bordeaux.” (The 2005 vintage in the Loire was one of the best in many years.)