The gentleman was wearing a beige jacket, shirt, and slacks. He was dressed respectably as though for a day in the office, and he looked to be in late middle age, with graying hair. If you were to try and identify the violent thug among the thousands of people standing that day by the roadside on the Puy de Dôme, he might have been the last pick.

He was just one member of an estimated 100,000-person crowd, who packed the 12 percent slopes of the 1,415-meter dome-shaped extinct volcano in France’s Massif Central. The Puy de Dôme is a climb with a unique place in Tour de France history, because of the events that would unfold on this day, and because it is a private road that has been used infrequently in the race. The man in the beige coat had paid to watch the stage, just like the hundred thousand other fans—it was one of the only times the Tour has charged a fee for roadside spectators.

Many had come to catch a glimpse of the great Eddy Merckx, who started the race in yellow, the 91st time in his storied career that he had worn the jersey, awarded to the rider leading the Tour de France. Many more were cheering for Frenchman Bernard Thévenet and rooting against Merckx. The Belgian rider had won five previous editions of the race and was on his way to a sixth, besting the record of Tour victories he then shared with legendary French cyclist Jacques Anquetil. But Merckx’s encounter with the man on the side of the road would shake up the standings, alter the course of his career, and highlight one of the most unique and defining traditions of the Tour de France.

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Although Merckx had endured a difficult season in 1975, it was only difficult when viewed in the context of the cyclist’s career. The sport had never seen anything like Merckx, a rider so dominant he had been given the nickname “The Cannibal.” In addition to his record five consecutive Tour victories, Merckx won 525 professional races, more than any other rider. That list included wins at 11 Grand Tours and 28 one-day Classics. Yet one of the enduring curiosities about the Cannibal was his mental fragility; he was racked by doubts and anxieties, particularly when it came to injuries and illness. Perhaps that had something to do with what unfolded in the 1975 Tour.

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It had been a turbulent year for the rider. It started strong with victories at Milan-San Remo, the Tour of Flanders, and Liège-Bastogne-Liège. But then a case of tonsillitis forced him to miss the Giro d’Italia, his preferred method of Tour preparation. Still, he began the race as the favorite, and with customary aggression, he attacked on Stages 1a and 1b (both held on the same day in June) and finished second on both. He was homing in on his sixth Tour de France win until July 10, 1975, on the Puy du Dome—Stage 14 of the 62nd Tour.

As Merckx strained up the Puy, propelling his bike along a narrow corridor of pavement between the fans, the man in the coat reached out violently—and in doing so breached a longstanding treaty between fans and professional cyclists. The thrilling, potentially dangerous proximity to fans and professional cyclists is one of the great allures of the Tour de France. On the mountain stages like the Puy du Dome and the Alpe d’Huez, hundreds of thousands of fans line the narrow roads, swelling on the street, blocking the route and waiting until the last crucial moment to part, leaving only a few feet or inches for nervous riders to slip through. These scenes, captured in photographs and on television, have become iconic moments that have come to define the Tour. In no other sporting event do such large numbers of spectators become integral components of the scene and the stage, nor do they have the capability to influence the outcome as they do during the Tour.

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That’s why the incident on Stage 14 reverberates down the years. Eleven Tours later, when Greg LeMond rode toward Alpe d’Huez in the company of his teammate-come-rival Bernard Hinault, it was the assault on Merckx that dominated his thoughts. “I was worried. I was thinking of Eddy Merckx,” LeMond told me for my book, Slaying the Badger. “I was thinking there could be someone out there. There was such strong feeling out there. It was so frenzied. And I’m racing against France’s best-known athlete…”

There were more echoes when Lance Armstrong faced a time trial, again on the Alpe, in 2004. A bit like Merckx in 1975, Armstrong was not exactly Mr. Popular in France. An assault would have been easy, too easy, but he feared worse: A death threat prior to the stage was taken seriously enough by the police for them to place an officer in a following car.

There have been other unpleasant incidents between fans and riders. In 2012, tacks were spread on the road in the Pyrenees. In 1992, there was a terrorist attack when the Tour started in San Sebastian, Spain, and Basque separatists bombed a car belonging to the British TV station Channel 4. These days, the threat of terrorism looms large but silently, barely mentioned. But with recent events in France, in Paris and Lyon, and the cancellation of the Eschborn-Frankfurt race in May after the discovery of bomb-making equipment in the home of a suspected Islamist extremist, the subject increasingly crops up in conversation, and must command considerable attention from those whose job it is to police the race and keep it safe.

Most fan-rider collisions are more like benign interventions. In 1999 on Alpe d’Huez, Guiseppe Guerini was taken out by a French student crouching on the road and peering at the Italian through the viewfinder on his camera. A similar incident had occurred in 1994, when a policeman, taking a photograph for a young girl 200 meters from the finish line in Armentières, misjudged the proximity of the rampaging bunch and was hit square in the midriff by Wilfried Nelissen, causing a horrendous pile-up. Famously, on Luz Ardiden in 2003, Armstrong—like Merckx on the Puy de Dôme, riding too close to the line of spectators—got his handlebars caught in a child’s bag.

“It was a little boy, probably a 10- or 11-year-old kid, whose parents had bought a commemorative musette from the concession stand,” Armstrong told me last year. “And he was kinda flapping it back and forth. And he timed it just right, caught my handlebars, and down I go.”

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Last year, the sheer volume of spectators in the Tour’s first three days in England, particularly in Yorkshire, created tense moments for the racers. Fans packed the roads for entire stages, 10- or 20-deep in places, spilling into the middle on climbs. On one stage, according to the German sprinter Marcel Kittel, the crowd parted at the last possible moment but left an elderly woman in a wheelchair in the riders’ path. The expanse of the crowds created other problems for the riders—where to stop to answer a call of nature—but a particular danger was the phenomenon of the “selfie,” which required spectators to stand with their backs to the action.

The frenzy of Yorkshire possibly contributed to defending champion Chris Froome’s crash on the first day back in France. He went down after a moment of inattention—maybe, thought Froome later, because his nerves were so frazzled after England that, back on the relatively quiet roads in France, he switched off mentally.

Always there is this danger, and questions over safety. And yet what would the Tour be without the crowds? Without the proximity? An estimated 15 million people watch the Tour by the roadside, close enough on climbs to touch the riders. Most do not. If they do, it is to offer a helpful push.

Watching fans at the Tour—many of whom have been standing for hours waiting, some drinking alcohol for just as long—is to be struck by the overwhelming decency (and the wisdom) of the crowd. It is a curious phenomenon to observe: How rowdiness dissipates as the race approaches, and then you see the crowds lean in, studying the faces and bodies of the gaunt and fragile-seeming athletes as though they are artifacts in a museum or valuable pieces in an art gallery, ever mindful that they should keep a respectful distance—even if it amounts to mere centimeters.

There is a boundary, enforced not by the gendarmes who stand at regular intervals and guard every junction, but through the restraint, respect, and self-policing of people who have simply come to watch and enjoy the world’s greatest bicycle race.

It amounts to a pact of trust between the people and the riders; the public and the Tour. Which is what made the incident in 1975 so shocking, so outrageous and unprecedented that it is still being talked about 40 years later.

It was on the upper slopes of the Puy de Döme, as Merckx thrashed the pedals in pursuit of Lucien Van Impe and Bernard Thévenet. Merckx hugged the right side of the road, almost brushing the line of spectators. He seemed to bounce off one and then, a few yards further on, lurched to the left as his hand shot to his abdomen. To people watching, it wasn’t clear what had happened. Merckx had been punched.

One of the questions as the 2015 Tour gets underway, on the 40th anniversary of the Merckx assault, is whether self-policing is sustainable or realistic. In the age of selfies and Vines, and of the instant “viral” fame visited upon anyone who shares screen time with the riders, can fans be trusted to honor the pact? Will modern phenomena—the camera phones and social media; the malignant threat of terrorism—change the relationship between riders and public?

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Striking the balance between drama and security is one of the Tour’s great challenges, and one I will be exploring, with riders, directors and organizers, as the Tour winds its anti-clockwise way around France on a route uncannily similar to the one followed in 1975. Over the next three weeks, through multiple installments of this story, we will also look back to that race and one of the Tour’s most infamous incidents—its dirty secret—to examine what happened, and its enduring significance.

CONTINUED: Who Punched Eddy Merckx Part 2