It is the kind of story that can cause a patriotic Frenchman to choke on his glass of pastis. The newspapers, bien sur, have splashed it across their front pages. Radio stations have been breaking into their programs to keep listeners up to date with the latest developments.

Quel choc! Quelle sensation! For the first time in the 73-year history of the world's most grueling bicycle race, an American has taken the lead in the Tour de France.

The name of the Tour's new "yellow jersey," or overall leader, is Greg Lemond. He has wavy blond hair and sharp blue eyes, he is 25 years old, and he comes from Sacramento, Calif.

To appreciate the impact of this news, you have to understand what the Tour means to France. It is more than just a 2,500-mile race over 23 days. It is one of France's great national obsessions, a kind of soap opera on wheels that provides the country with something to talk about during the torpid days of July.

Winding its way from the Atlantic to the Alps before a final sprint up the Champs Elysees in Paris, the Tour generates enough emotion to fill a TV miniseries. There are stories of heroism and skulduggery, ambition and rivalry, overnight fame and equally sudden bad luck. This year has been dominated by talk of fratricide, with Lemond pitted against his more famous teammate, Bernard Hinault, a five-time overall winner of the Tour.

If Lemond wins the race Sunday, as he is now well placed to do, it will be somewhat akin to a Frenchman running away with the Heisman Trophy. No non-European has ever won the Tour de France, which has been dominated by French and Belgian cyclists since its inception in 1913. Until this year, the American riders were regarded as perennial also-rans whose function in the race was to help French superstars like Hinault.

"It's fantastic, the Tour de France is a test of strength that resembles nothing else in the world," Lemond told reporters yesterday after gaining the overall lead. "But I know that 50 million French people are willing Bernard Hinault on to win his sixth tour. I am up against a terrible nationalism."

The course of the Tour changes every year to allow people living in different regions of France a chance to watch the cavalcade of riders, journalists, officials, and commercial sponsors whiz past in a blur. This year's race is widely considered to be the most challenging in over a decade, with the number of competitors down to below 140 out of an original pack of 210 who left the suburbs of Paris two weeks ago.

FOR THE PAST two days, the cyclists have been battling it out in the Alps -- the mountainous region of southeastern France where Tours tend to be won and lost. A slight wavering by Hinault as he went over three successive 7,500-foot peaks on Sunday allowed Lemond to grab the coveted yellow jersey from his 31-year-old teammate.

Lemond's achievement was hailed by front-page headlines like "The American Challenge," "Lemond Dethrones Hinault," and "The First American to Wear Yellow." The newspaper L'Equipe, which is to French sports fans what the Wall Street Journal is to American bankers, described the day as a "historic event" that would serve the cause of world cycling by promoting the sport in the United States.

The rivalry between Lemond and Hinault provoked stories about a "Cain and Abel" feud within the Vie Clair team. Public statements by the two men praising each other's generosity and sportsmanship have alternated with offhand remarks suggesting that relations between them are seriously strained.

Yesterday, in an apparent attempt to dispel reports of a rift, both riders crossed the finishing line together, their arms linked in a victory gesture. Hinault was technically the winner of this 18th stage, after Lemond pushed him forward over the finish line, but the American retained an overall lead of 2 minutes, 45 seconds over his teammate.

Adding spice to the personality conflict is the fact that the highly successful Vie Clair team is owned and managed by an entrepreneur, Bernard Tapie, who makes no secret of his wish to penetrate the American market for sporting goods. Last year alone, he was able to sell about 250,000 cycles on the basis of Hinault's name, and he could do even better this year if Lemond wins.

Tapie's multimillion-dollar operation, which includes the promotion of diet foods, illustrates how the Tour has changed since the early years, when each rider raced for himself. In recent years, the race has become extremely commercial, generating huge profits for the winning team. This in turn led to a system in which individual team members were expected to sacrifice themselves for the team leader.

In a new twist this year, Tapie has described both Hinault and Lemond as "coleaders" of the Vie Clair team, both cooperating with and competing against each other. In the opinion of Jacques Goddet, a commentator for L'Equipe, the result is a kind of "fratricidal psychodrama."

As far as the French fans are concerned, Hinault is the clear favorite. Wherever he appears, he is urged on by chants of "Allez Hinault" (Go Hinault), while the American rarely rates more than polite applause.

Known in the French press as "Gentleman Greg," Lemond first made his mark on the cycling world in 1983 when he won the world championship in Switzerland. His annual salary from Vie Clair plus income from endorsements is said to be in the region of 7 million francs ($1 million), an astronomical sum for a cyclist. Until now, however, he has failed to live up to his early promise.

Some French sports commentators say that Lemond lacks the ruthless determination to win that characterizes the real champion and that he has never been more than a second string to Hinault. His performance this week suggests that he is anxious to shake off that reputation.

Hinault, on the other hand, clearly prefers the one-team, one-leader system, despite a reported promise that he would help Lemond win the Tour this year. Asked about this week's upset, he replied simply: "Greg is the yellow jersey and I am the captain of the team. It's a long road ahead to Paris."