The film starts playing with expectations (and morbid ones, at that) from the first shot, as we see Lloyd behind a bars, his mother and a clergyman bidding him good-bye, as a policeman looks at his watch while a noose hangs in the background. No, he's not about to die. He's leaving home, the bars to keep people from crossing over onto the tracks and the noose a delivery system for pass-by messaging. But the camera trickery and stunt-work—at about 50% Keaton speed—is very much in evidence right from the beginning, and does not flag throughout the film for its 70 minute running time, up to and including a sequence so iconic—with effects trickery so effective—that it stands as one of the great representations of the silent era.
Lloyd's "Boy" (his pay-checks come to him as Harold Lloyd, however) after weeks of making do and hiding from his landlady, manages to land a job at the fancy-schmancy DeVore department Store as a counter boy. His letters home paint a different picture, saying he's a successful manager, and though he keeps trying, it always seems that something—including the snooty floorwalker of the store, Mr. Stubbs (Westcott Clarke)—gets in his way.
Then, disaster strikes. His girl comes to visit and he has to work overtime trying to impress that he's "big" at the store, then he overhears the general manager wracking his brain how he can get more bodies to the store (which doesn't appear to be a problem as the counters are always over-crowded with fighting customers). Harold says he'll climb the side of the building like a human fly for enough money to stake a wedding, and puts his room-mate—a high-rise construction worker—up to the task.
Last minute complications set in, as the room-mate is trying to run from the law, so it's up to Harold to climb the building, but only for one floor until they can switch clothes and his buddy can take it the rest of the way.
Doesn't happen that way. It's still an impressive sequence, no matter how—and there is still some conjecture about that—they did it. But between Lloyd and stunt man Robert Golden the sequence is hilariously nerve-wracking, constantly inventive, and keeps building complication upon complication. it is deservedly one of the most remembered films of the silent era (and not just for the last twenty minutes), and made it into the National Film Registry in 1994, five years after they started making the selections.