"Once Upon a Time" is episode 78 of the American television anthology series The Twilight Zone. It originally aired on December 15, 1961. It features early film star Buster Keaton in one of his later roles, as an unlikely time traveler, and the opening and closing scenes pay tribute to the silent films for which he was famous.

Joseph Frank "Buster" Keaton was an American actor, comedian and film director. He is best known for his silent films during the 1920s, in which he performed physical comedy and inventive stunts. He frequently maintained a stoic, deadpan facial expression that became his trademark and earned him the nickname "The Great Stone Face".