Here’s a film that brought to my mind Mary Poppins’s famous order: “Close your mouth please Michael, we are not a codfish.” I spent most of this film with my jaw on the floor. It’s a visually staggering documentary from National Geographic about the climber Alex Honnold, who specialises in the most mind-boggling and gasp-inducing “free solo” climbs – without a rope, up sheer rock faces, the sort of thing usually undertaken by Tom Cruise or Spider-Man.

This film is about Honnold’s freakily dangerous free solo attempt in 2017 at El Capitan, the 3,000ft-high rock formation in Yosemite Valley, California. We see his difficult relationship with his girlfriend Sanni McCandless. Like all those close to Honnold, she is in the purest form of agony as Honnold makes his almost supernaturally difficult climb. Why is he doing this crazy thing? The nearest Honnold comes to a because-it-is-there moment is comparing his physical and mental intensity to that of a samurai.

The documentary also takes in the views of the camerapeople who are recording his climb – climbers themselves, going up ahead of him or behind him (with ropes) or sometimes deploying drones. Has this camera crew thought, really thought, about what they would think and feel if the worst happened? Because there are plenty of examples of free soloists who have died. Do the camera operators realise they would be intimate witnesses to a catastrophe? Have they thought how they would film it – do they let Alex drop out of the frame or, if positioned above, watch him plummet? And then what do they do with the film? There is a heartstopping footage of someone appearing to fall, and it isn’t entirely clear who that is.

Alex Honnold himself is an enigma: equable, even-tempered, but withdrawn, although McCandless clearly wants him to be more emotionally open with her. As for Honnold, he clearly isn’t finished with free solo climbs. Does a samurai’s destiny await him?