Their career trajectories were moving inexorably in opposite directions: Marchand, having relocated to Auburn in hopes of reaching a third Olympics that summer, would fall short of that goal and soon retire. Phelps, guided by coach Bob Bowman, would win six gold medals at the 2004 Athens Games, the starting point for the most decorated Olympic career in history.

Some two minutes after launching themselves into the water, Phelps touched in first place, three seconds ahead of the runner-up. Marchand was a distant sixth, nearly nine seconds behind Phelps.

Now, two decades later, consider the cosmic circles — great circles of life, of fate and of history — coming closed this summer at the 2024 Paris Olympics, which fall on the 100th anniversary of the watershed 1924 Paris Games.

Marchand, now 50, married another French Olympian, Celine Bonnet, and had a son, Leon — who, despite his parents’ reservations, born of a shared, lived appreciation of the sport’s brutal demands, eventually found his way into the same chlorinated waters. Though he would quit the sport for two years at age 7, saying the water was too cold, he would return — this time to stay.

Like his father, Leon Marchand would find his way to America. In 2021, at 19, he would reach out to Bowman — Phelps’s former coach — via email and soon join Bowman’s nascent Arizona State program, winning 10 NCAA titles before he was done.

A year later after arriving in the United States, Marchand would become a world champion in his father’s best event, the 200 IM, and claim a second gold in the longer version, the 400 IM.

A year after that, at the 2023 world championships in Fukuoka, Japan — with Phelps in the building as a commentator for Peacock’s streaming broadcast — Leon Marchand would obliterate the legend’s last remaining world record by nearly a second and a half, winning the 400 IM in 4 minutes 2.50 seconds, one of three gold medals earned by Marchand at that meet.

“Uh-oh,” Phelps said into his microphone that day as Marchand turned at the wall for the closing freestyle leg. And a few moments later: “It’s gone.”

Beginning Saturday at Paris La Défense Arena just outside of the French capital, with his Olympian parents in the stands, neither of whom reached the podium in their combined three Summer Games, the younger Marchand, 22, will climb the starting blocks as arguably the dominant male swimmer in the world, a gold medal threat in four individual events and the face of the Paris Games.

“I think I’m well prepared for it,” Leon Marchand said during an interview this year when asked about the massive expectations and pressure on his shoulders at an Olympics in his home country. “It’s going to be a factor, for sure. It’s not going to be only about swimming now. … My well-being is essential.”

As for Xavier Marchand, an Olympian at Atlanta 1996 and Sydney 2000, he said he believes Leon inherited his mother’s suppleness and leg strength. “And I’d say he’s taken my mind,” he said in an email exchange, with his answers written in French, then translated to English.

Asked whether he recalled racing against Phelps all those years ago, he could summon no specific memories of his sixth-place finish in Auburn in 2004, which is believed to be the only time they raced each other in a 50-meter, Olympic-size pool.

“At the time,” Xavier Marchand said, “I didn’t imagine — and I don’t think many (other) people did — that he would become the greatest champion in history.”

Strangely, though, he has a much clearer memory of racing Phelps at a World Cup meet in Stockholm (in a 25-meter pool) in 2001 — when he was 27 and Phelps was 15 — and beating the future legend by nearly a second. Xavier Marchand’s email referencing that race, details of which were confirmed by a search of World Aquatics results, contained two emojis requiring no translation: flexed biceps and rolling on the floor laughing.

Bowman received an email like dozens of others in his inbox every month from prospective trainees looking for a spot on his elite roster. But the name on this one jumped out: Leon Marchand.

“I was like: ‘Marchand? Could this be Xavier Marchand’s kid?’” Bowman said, recalling the graceful French swimmer of the previous generation who stood out for his prominent sideburns and powerful breaststroke leg. “And sure enough, it was.”

It was late spring 2021, and Bowman was in the process of building his Arizona State program into a powerhouse that would win an NCAA men’s championship three years later, with Marchand as its cornerstone. But now, in as much time as it took Bowman to do a quick scan of the young Frenchman’s times, he sent back a reply: “Yes, we would be very interested in having you.”

The email to Bowman was one of many sent out that week by Marchand, who had decided to train in the United States because of the superior competitiveness and structure of its NCAA system and who appeared to be headed to California Berkeley until the school suddenly informed him it couldn’t offer him a full scholarship. Forced to reopen his search, Marchand hadn’t expected to find his perfect match so quickly.

“What tipped the scales,” he would tell French reporters in 2022, “was the coach.”

Bowman and Marchand carried on a WhatsApp conversation for the next several months — with the coach delighting his new charge by passing along some of Phelps’s training sets to try at home. The first time they met would be in 2021 at the pandemic-delayed Tokyo Games, where Bowman served as a Team USA assistant coach and Marchand was a 19-year-old first-time Olympian who was just starting to gain recognition as a contender in the IMs. Marchand would compete in four events in Tokyo, with a best finish of sixth in the 400 IM.

“We hugged right away,” Bowman recalled, “like we had known each other forever.”

It was evident immediately to Bowman that Marchand had none of Phelps’s obvious physiological advantages, which included a 6-foot-4 frame and a wingspan said to be 79 inches. At 6-2 and 170 pounds, Marchand was neither exceptionally tall nor particularly strong and broad-shouldered.

But once, when the French national team conducted “passive drag tests” on their swimmers — placing a tether around their torsos and dragging them, in a basic and stationary streamline position, through a 50-meter pool — Marchand blew away the lowest score team officials had ever recorded. In essence, he was built to glide through water.

“He’s built like a torpedo,” Bowman said. “His hips and shoulders are the same width. That really helps.”

Bowman wasn’t trying to create a chlorine-fueled clone of Phelps, knowing better than most that the latter’s combination of physiology, versatility and capacity for high-intensity, 10-times-a-week training probably would never be seen again. Their skill sets were different — Phelps was more of a freestyle/butterfly sprinter by nature, while Marchand’s best individual stroke is probably breaststroke — but their talents merged in the IM.

The biggest similarity Bowman noticed was in the way they practiced. Once, at the end of a set of 500-yard freestyles, with Marchand hanging on the wall trying to catch his breath, Bowman challenged him to do one more — with the stopwatch running this time. Marchand asked Bowman what Phelps’s fastest time was in that race. Bowman told him — and Marchand beat it by five seconds. This past spring, at the NCAA championships, Marchand took gold in that race despite the freestyle being his third-best stroke.

“Leon has done some practices that I’m pretty sure no one else in the world has ever done — just like Michael would do,” said Bowman, who left Arizona State this year to coach at Texas, where Marchand is expected to join him. “But where he’s most like Michael? On a scale of 1 to 10, where 1 is terrible and 10 is the best you can humanly do — Leon is like an 8.5 every day. He’s never a 2. He’s seldom a 10. But a high average every time.

“And that’s what Michael did, year after year after year.”

The breaking of any world record feels momentous in real time, a recalibration of the outer limits of human capability. But Marchand’s takedown of Phelps’s 400 IM standard in Fukuoka seemed downright tectonic. Not only was it Phelps’s last world standard, it had been the longest-standing world record in the sport. Phelps set it at the 2008 Beijing Olympics, at the height of his unprecedented powers. But he actually had held the record since 2002 — having rebroken it seven more times — making his reign the longest in the sport since World War II.

Japanese media began referring to Marchand as “Shin-Kaibutsu” — “New Monster” — a nod to the “Kaibutsu” nickname by which Phelps was known there.

Almost immediately, Marchand recognized the magnitude of what he had done — and the implications for the future. With Paris 2024 a year away, his life had just become more complicated, more public. The expectations on his shoulders grew exponentially.

“I realized when I broke Michael’s record: ‘Well, the Paris Games are going to be different from what I imagined,’” he told French reporters this year. “I’m preparing myself mentally and physically for this.”

Luckily for Marchand, he won’t have to navigate the Olympic circus alone. In Bowman, who joined the French coaching staff this summer, he will have at his side arguably the greatest such navigator in history, having guided Phelps through the madness at four Olympics. Bowman’s first move when arriving at the Olympic venue, he said, was to locate the side or back exits, so he would be able to direct his superstar swimmer away from the chaos on and around the pool deck.

“I’m very good at saying no,” Bowman said. Of Marchand’s upcoming circus debut, Bowman said: “He has an idea (of how suffocating it will be). But until you live through it, you don’t really have an idea.”

While Marchand’s prowess in the IMs has made him an international superstar, swimming insiders are perhaps even more excited about the other events he is expected to swim at the Olympics: the 200 butterfly and the 200 breaststroke.

Not only has no swimmer ever won both events in the same Olympics, no swimmer has medaled in butterfly and breaststroke races of any distance in the same Olympics. Of the handful who have attempted both strokes, only American Mary Sears, in 1956, even managed to make the finals in both, earning bronze in the women’s 100 fly and finishing seventh in the 200 breast.

As for Marchand, his path to a historic double victory is daunting. In both races, he will have to vanquish the reigning world record holder: Hungary’s Kristof Milak in the 200 fly and China’s Qin Haiyang in the 200 breast. And because the events are on the same day, he will have to swim both the 200 fly and 200 breast in the preliminary heats on the morning of July 30, then swim both again in the semifinals that evening — and, assuming he advances, yet a third time in the evening finals July 31.

“There are not many people who try this double. It’s a bit weird,” Marchand acknowledged. “But I like everything weird.”

Not so long ago, such a thing scarcely seemed possible. In 2017, when the International Olympic Committee announced Paris as the host city for the 2024 Summer Games, it barely registered a shrug out of Marchand. At 15, the same age when Phelps made his first Olympic team, Marchand was merely a solid swimmer in the southern city of Toulouse. Few considered him a phenom, let alone a future superstar. That summer, Marchand ranked 31st in France in the 400 IM and 78th in the 200, and he failed to make the national team for the world junior championships.

“At the time I was not really good at swimming … so I don’t think (Paris 2024) was really an idea in my mind,” Marchand said this past spring. “I think my technique was really good from the very beginning when I was young. But I was not really a racer; I didn’t want, really, to win. I was just, like, swimming every day a little bit to have fun.”

Imagine everything it took — the genes of his Olympian parents, the drive and ambition to relocate half a world away, the twists of fate that put his career in the hands of Phelps’s former coach — to propel that non-racer to where he is now, with an entire nation resting its gold medal hopes on his narrow shoulders. In the history of Olympic swimming, France has produced just seven individual gold medalists, none of whom won multiple golds.

The many thousands who pack Paris La Défense Arena will expect nothing less than the sight of Marchand atop the medal stand at least a couple of times, the strains of “La Marseillaise” filling the building, the parents weeping in the stands, the next great circle coming closed.