The attack on the World Trade Center on Sept. 11, 2001, is never mentioned in “Man on Wire,” an omission that seems initially curious but eventually makes sense.

This mesmerizing documentary is not about Sept. 11, 2001, but about what happened at the World Trade Center on Aug. 7, 1974. On that morning, the French artiste-daredevil Philippe Petit staged a mind-boggling stunt: Petit walked from the roof of one tower to the other (and back again, for 45 minutes), balancing on a wire.

The film, directed by James Marsh (who made the interesting fiction feature “The King”), gives the background on this amazing event. Petit and a group of friends planned the stunt for many months, traveling to the towers with false credentials, taking pictures at the top, creating miniature models of the buildings.

Petit had already performed high-wire exploits at Notre Dame cathedral in Paris and on a bridge in Sydney. As he recounts in the movie, he saw a drawing of the World Trade Center in a magazine even before the buildings were erected, and instantly imagined himself tightrope-walking between them.

Petit and his co-conspirators actually filmed themselves during their preparations (which, as one of them notes, was “like a bank robbery”), as they argued strategy and practiced the walk.

Re-creations give a flavor of what it must have been like to lug their equipment into some empty floors near the top of the WTC on the night before the big walk, dodging security guards and waiting nervously for dawn. One group was in each tower, ready to string the wire across to each other.

You can’t help thinking of the eerie subtext of 9/11 through all of this, even though these conspirators had a benign plot in mind.

It helps that Petit and the others are all cheerful and expansive, and that Petit himself constantly speaks of the event as a work of art, a poetic gesture toward sky and space.

If you suffer fear of heights, “Man on Wire” will have you in a sweat. But also enthralled. Monsieur Petit is a very special kind of madman.

Of all the testimonials in the film, none is better than the on-the-spot report of an eloquent New York cop who tried to arrest Petit while the wire-walker was in midair. “Everybody was spellbound in the watching of it,” the cop concludes. You can’t say it better than that.