I turned and saw Nikita climbing a drain pipe to the roof of one of the stores. While I was writing “Spider-Man” in my notebook, his shadow as he leaped from one roof to another passed over the page.

Jeff Belle has an office in Paris, where he handles fire-department business. To speak with him, I brought a translator, Susan Chace, an American novelist who lives in Paris. Belle, who is forty-five, is small and wiry. He has a round face, a sharp chin, dark eyes, and black, cropped hair with flecks of gray. He was wearing a fireman’s uniform, a dark-blue garment like a jumpsuit. The legs ended just above the tops of his black combat boots. Over the uniform, he wore a long coat that had “Pompier de Paris” written on the back. We met at his office, then he took us to an empty, low-ceilinged room with a bar and an espresso machine and some tables stacked on top of each other. On the wall were photographs of firefighters.

Jeff said that David was a restless boy. “He was always exercising in front of the TV,” he said. “He still takes whatever’s next to him, maybe a big book, and starts lifting it. He can’t sit still. He lives with it.” The brothers did not see much of each other until David, at fourteen, moved to Lisses to live with his mother. Then Jeff, who was already a fireman, began to look after him. He would show him how to climb ropes and perform gymnastic maneuvers, and David would go off and do it his own way. Now and then, David would go to the climbing wall in Lisses with his father and show him things he had taught himself, and Raymond, thinking that he was being encouraging, would say, “I could do that when I was nine.”

Through Jeff, David was exposed to the methods of Georges Hébert, a French sports theorist, whose motto was “Be strong to be useful.” Hébert believed that modern conveniences such as elevators were debilitating. He thought that Africans he had met while travelling were healthier and stronger than Europeans, and that the proportions of the bodies he saw in Greek and Roman statues were ideal. The philosophies and the exercises he developed, which are part of a French fireman’s training, were also meant to cultivate courage and discipline. Inspired by Hébert, a Swiss architect developed an obstacle course called a parcours. “David took Hébert’s ideas and said, ‘I will adapt it to what I need,’ ” Jeff said. “Instead of stopping at a reasonable point, he just kept going.”

David was briefly a fireman recruit, until he hurt his wrist. While he was recuperating, he started thinking things over and saw that the life of a fireman had too many rules, and not enough action, and he decided to join the Marines, but he didn’t find the same values among them, the “traditional values.” He left the Marines and went to India, where he stayed for six months. When he came back to Paris, he was twenty-four, and he didn’t know what to do with himself.

“He came to see me at my house,” Jeff said, “and he told me he didn’t know where his life was going. He was only interested in parkour. You could be a super policeman or a firefighter using it, but you can’t earn your living, because there’s no championship. I said, ‘Maybe if we film what you’re doing.’ ”

It was 1997, and Jeff was involved in planning an annual ceremony in which recruits perform firefighting drills. He decided that David should put on a show. He told him to get a group together, so that he wouldn’t look insignificant by himself. David collected two of his cousins and some other kids from the neighborhood, including Sébastien Foucan, with whom he ran around doing parkour. Jeff choreographed a routine for them. They dressed as ninjas and called themselves the Yamakasi. “It means ‘strong spirit’ in the language of Zaire,” Jeff said, “but it sounded Asian.” During the show, David climbed a tower and did a handstand at the top. He also scaled a fireman’s ladder and did a backflip from it. After the demonstration, David began getting invitations to perform.

Jeff is proud of David, but worries about him. “This was a kid who refused any kind of system, who just wanted to live his life,” he says. “If he’s surrounded by the right people, he can do what he wants. Ordinary life really upsets him, though, because this world the rest of us live in is not where he finds his pleasure. He’s easily disturbed by ordinary things. But he’s also asking, ‘Why am I doing this parkour?’ All his family who did this physical stuff were doing it for a reason, but he’s asking, ‘Why am I doing this, what does it mean?’ ” Jeff added, “He’s simple in his purposes. He doesn’t like talking very much. He’s someone who is looking for his way.” I asked what sort of routines David observed in his training. Jeff shook his head. “He’s still eating Big Macs and drinking Coke,” Jeff said. “He likes chicken sandwiches. He trains when it comes to him. He’s usually sleeping in the morning. He’s really a night guy.”

We arranged to meet with David the following day, in Lisses, where he was staying with his mother. When David is in France, he lives either with her or with Jeff. “He doesn’t really have a lot of money,” Jeff said, “although people think that he does.” He added that David was very easy to live with. “We don’t know about the inside of his head, but outside he’s very neat. His room is always in perfect shape.”

The next day, I took a taxi with Susan Chace to Lisses, about half an hour south of Paris. When we were under way, the driver asked why we were going there. Chace said, “To see David Belle.” The driver nodded. Chace asked if he knew who Belle was, and the driver said, “Of course. Il est unique.” We left the highway and, following Jeff’s directions, went around a rotary and came to a collection of low, flat-roofed, two-story buildings, like shoeboxes, painted light brown. We stopped in front of Belle’s building. Chace knocked, the door opened, and the driver said, “That’s him!” Belle had his chin tucked slightly, like a man looking out from under the brim of a hat. He had dark hair cut short like a pelt and a thin, asymmetrical face, with a sharp chin and a hook nose. He was wearing a red fleece top and jeans. As I paid the driver, Jeff Belle drove up behind us. We went into the apartment. The kitchen was by the door, and there was a living room beyond it with a circular stairway leading up. In the living room was David’s girlfriend, Dorine Sane. David had his fleece top zipped to his chin—he had a sore throat—and he seemed subdued. He had just come back from three weeks in the Czech Republic, where he was making “Babylon AD,” which stars Vin Diesel, and is based on a French science-fiction novel. David plays the head of an Internet gang that does parkour, and he choreographed scenes for ten actors.

Jeff and David spoke for a few moments, and then Chace said that David was going to rest, while Jeff took us to the Dam du Lac. Outside, we crossed a parking lot, then took an asphalt path through a park. Several hundred yards off, the Dam du Lac rose up against the side of a small lake. It was the color of sandstone and had the shape of an arch. “David was afraid of it in the beginning,” Jeff said. “Now he walks on it like it was solid ground.” The lac turned out to be made of concrete. As we walked along the edge, ducks paddled away from us. For some time now, a fence has enclosed the wall, but it was easy to climb around it. The wall was slightly concave, and the top was intersected by a horizontal slab, which had roughly the dimensions of a king-size mattress and was curled up at one edge. Here and there on the face of the wall were footholds and handholds in the form of slots the size of bricks. On one side was a rectangular box, open at one end, like a cave, which is called the cabana. Below, sticking straight out from the wall and about fifteen feet tall, was a form in the shape, more or less, of a hammer. Jeff said that the first maneuver David had done was a backflip from the cabana to the hammer. About twenty feet from the ground was a sign saying, “Escalade Interdite.” On it were signatures. “The kids climb up and sign their names,” Jeff said. “David also went barefoot on it.” He pointed at the top. “And at night sometimes he slept up there.”