Richard Schickel: "The impeccable comedian directs himself in an impeccable silent comedy. The man with the flat hat and the dead pan has a night job as a movie theater projectionist but daydreams about becoming a famous (and natty) master detective. In real life he is falsely accused by a shameless cad of stealing a watch from his girlfriend's father. At work that evening he sleepwalks himself into the film he's projecting (its plot eerily mirrors his real-life problem) and solves the crime in a series of magnificently imaginative, physically perilous, perfectly orchestrated gags."
Wikipedia: "Into the film: Keaton "walked" into the movie via the power of suggestion. The scene shifted back and forth several times from the projectionist's booth to the movie that was being shown. But for the last shift, instead of showing the movie, the camera this time showed a stage with live actors, designed to replicate the look of the movie. Therefore, Buster actually climbed onstage, but created the illusion of joining the movie. It wasn't until the 1940s that Keaton revealed that he and his cameraman had used surveyor's instruments to position him, and the camera, at exactly the correct distances and positions to provide the illusion of continuity."