On Aug. 6, 1974, a French-American team snuck into the uppermost floors of the World Trade Center towers with one ton of equipment and worked into the early hours of the next day, stringing a cable between the two rooftops. Early on Aug. 7, ignoring every instinct of self-preservation, a man named Philippe Petit stepped onto the wire and walked between the rooftops for 45 minutes. Police officers gathered at both ends of the wires and onlookers amassed below. They were staring at a man who was 1,300 feet off the ground.

Based on Petit’s book, “To Reach the Clouds,” “Man on Wire” takes its name from a police report filed after his capture, when baffled officials tried to classify what laws he had broken. The documentary, directed by James Marsh, unpacks and lays out what led up to these three simple words. Marsh inter-cuts present-day interviews with Philippe and his crew, archival footage, filmed reenactments and surviving photographs and documents to retell the story of the preparations, the act itself and, briefly, its aftermath.

While Philippe is the star of the proceedings, both past and present, Marsh interviews many of his collaborators as well. They recount their reactions to hearing about Philippe’s plan and the nerve-wracking details of its implementation, from several near-misses with security personnel to shooting an arrow from one tower to the other. The individual roles of many of the players can get lost in the big picture and who winds up in each tower playing what role can appear beside the point. Only Philippe’s best friend, Jean-Louis, and his girlfriend, Anne, have a chance to express themselves in any great degree. Their emotional investment in Philippe’s dream is touching, and no less illuminating are their evident wounds at Philippe’s past behavior. The film makes it clear that those closest to a man with a self-absorbed confidence and a manic determination do not go unaffected. These close looks are fleeting, however, and for the most part the group forms a loose confederation, a chorus of reasonable people who assisted a dreamer. The film defines them in relation to him.

This isn’t a problem, however, because Philippe is probably the best interview subject a documentary filmmaker could imagine. He explains his dream to walk between the towers with a passion that convinces you of its importance by his fervor alone. He is lively and captivating when he recounts the details of the plot, and he speaks animatedly about the lucky strokes that allowed for success. The role of talking head cannot contain him. At times, Marsh has him get out of chair and actually act out what happened, not content merely to articulate but to show. Philippe remains every inch a performer, and a great one at that.

With this spark at the movie’s heart, the energy never ceases. The use of dramatic re-enactments, though irritatingly not separated from archival footage, gives the preparations the unhealthy excitement of a good caper. The information about planning and preparation doesn’t rob the walk of any of its wonder. It helps that the events are in no dispute and that the act is unimpeachably impressive — a great situation for any documentary — but Marsh also never loses sight of its core fascination.

Although the film commemorates an act of supreme recklessness, “Man on Wire” embodies restraint. There is no over-reach in assessing the importance of the event, and even Philippe is content to let the triumph of the action mostly speak for itself. Also, to the film’s enormous credit, there is no attempt to thread in the events of 9/11. “Man on Wire” keeps its aims humble, and it is content to leave you filled with a sense of possibility. This is a quiet gem that has slipped its way in between the summer’s more imposing projects, and it would be a shame to let it pass by unnoticed.