The 1924 silent film featuring Buster Keaton as a film projectionist and detective is Sherlock Jr.
Keaton plays a movie projectionist who dreams of becoming a detective. After falling asleep while screening a mystery film, his dream self enters the picture as Sherlock Jr., a brilliant investigator pursuing a stolen necklace and rescuing the heroine.
The film is celebrated for turning cinema itself into a source of comedy. Its projectionist moves from the booth into the projected story, while Keaton uses precise editing and in-camera effects to create startling changes of place and identity.
Sherlock Jr. is often confused with Keaton’s The General, The Navigator, or The Cameraman, but those films feature entirely different characters and settings. The film was selected for the National Film Registry in 1991 and entered the U.S. public domain in 2020.