PARIS — The 4-year-old boy seemed to be suspended from a balcony. An adult standing on a nearby balcony seemed powerless to help. Disaster seemed the only possible outcome.

Then, to the nimble rescue on the streets of Paris on Saturday evening, came a young man whom some French people have started to call the Spider-Man of the 18th, referring to the arrondissement of Paris where the episode unfolded.

With a combination of grit, agility and muscle, the man hauled himself hand over hand from one balcony to another, springing from one parapet to grasp the next one up. A crowd that had gathered before he began his daring exploit urged him ever upward, according to onlookers’ video that was shared widely on social media.

Finally, after scaling four balconies, the man reached the child and pulled him to safety. And suddenly, an act of individual courage and resourcefulness began to play into Europe’s fraught and polarized debate about outsiders, immigrants and refugees.