Dimension X was an influential science fiction radio program that aired on NBC from April 8, 1950, to September 29, 1951. It was one of the first adult-oriented science fiction radio programs, known for its dramatic half-hour format and anthology style that featured adaptations of stories from renowned science fiction authors. "Knock" is the fifth episode of Dimension X, originally aired on May 6, 1950. Based on a short story by Fredric Brown and adapted by Ernest Kinoy, the episode begins with the chilling line, 'The last man on Earth sat in a room. There was a knock on the door.' This intriguing premise sets the stage for a suspenseful tale that explores themes of isolation and the unknown. The episode's standalone story structure and gripping narrative exemplify the show's penchant for eerie and thought-provoking science fiction.