Buster Keaton stars here as a projectionist who wishes to be a detective and as a result is unable to perform adequately in either capacity. His detective skills are limited to the contents of his "How to be a Detective" manual, which his rival uses to frame Sherlock for stealing a watch. Unable to solve the case of the missing watch, Sherlock returns to the theatre heartbroken and falls asleep on the job. But while sleeping his dreams take over, and what dreams they are. He outwits the criminals, solves the case, wins the love of his girl, but this is nothing for Sherlock, Jr., the world's greatest detective.
The story is nice, but essentially this film is on the list for the dream sequence, the type of thing that makes you sit up and go, "I had no idea anyone did that in 1924." Keaton falls asleep and his dream alter ego wakes up in a double exposure. He goes down into the audience, where the characters in the film have been replaced with Sherlock's rival and love. Sherlock then walks into the picture.
In a remarkable scene, considering the time, the surroundings change from steps to a bench to a hill in the desert to the African safari to the ocean to a snow bank, only Sherlock stays constant in the frame. So when he jumps into the ocean, the edit comes while he is mid-air and he ends up planted in the snow. The whole thing is seamless, and reportedly done with nothing more than surveying equipment and the naked eye.
From there he is thrust into a mystery fraught with peril. The criminals replace the 13 ball on the pool table with a replica bomb and our hero manages to clear the entire table without touching the 13, unaware of the impending danger. Oh, but is he? Could it be possible that a detective as great as Sherlock, Jr. has noticed the switch and is using the bomb to learn the identities of the crooks?