When reporters met Philippe Petit after he’d spent 45 minutes on a tightrope he and his team had covertly rigged between the Twin Towers of the World Trade Center, they had an urgent question for him: why the hell would he do such a thing? Speaking in a superb new British documentary about that most singular achievement on a morning almost exactly 34 years ago, Petit still seems plenty piqued to hear such a banal query. “I did something magnificent and mysterious,” he says in a tone as quintessentially Gallic as soft-ripened Camembert. “I didn’t have any why.”

It’s to the film’s great credit that Man on Wire isn’t hung up on that question, either. Instead, James Marsh’s film — a prizewinner at Sundance and one of the most talked-about titles at Hot Docs — engages most fully with the matter of how he did it. A surreal yet true story of ambition, ingenuity and chutzpah, Marsh’s movie also turns out to be the year’s most riveting thriller so far.

As for Petit, he may be its most richly memorable character. After beginning his career as a street juggler in Paris in the ’60s, this irresistibly charismatic if inevitably narcissistic figure swiftly graduated to larger schemes. His early stunts as a tightrope walker — though, as many people note here, what he did was closer to dancing than walking — included a similarly clandestine walk across the towers of the Sydney Harbour Bridge in Australia. But he claims that thoughts of the World Trade Center dominated his imagination long before the buildings were even completed in 1973. Says one friend, “It was as if they’d been built for him.”

Using a blend of interviews, archival footage and recreations stylishly shot in black and white (Michael Nyman’s cyclical musical figures serve as the score), Walsh cuts between the daredevil’s colourful history and the immediate build-up to the walk. Many months in the planning, the latter unfolds as part big heist, part Jacques Tati farce. The sheer logistics of securing the cable were absurdly daunting even if the mission didn’t involve smuggling nearly a tonne of equipment past the buildings’ security guards. Equally bizarre is Petit’s coterie of collaborators, which included a New York musician who was deemed too much of a stoner to be entrusted with any of the important tasks. No wonder one of them worried that what they were really doing was participating in an “assisted suicide.”

Such was not the case. Petit’s big moment in the sky — which his folks on the ground helpfully pointed out to passers-by lest they miss it — would be embraced as one of the most magical pieces of New York lore. It would also help generate affection for the Twin Towers, then regarded by most Manhattanites as unglamorously utilitarian. That Marsh’s film makes only the slightest reference to the ultimate fate of the World Trade Center initially seems like an ethical lapse. But perhaps it’s more appropriate to regard this story as something of a fairy tale anyway, one whose incredible events existed out of time and somehow escaped the prosaic demands of gravity and reality.