Featuring "adventures in time and space told in future tense", Dimension X aired over NBC from April 8, 1950, through September 29, 1951. The series adapted stories by the modern masters of science fiction, including Ray Bradbury, Robert Heinlein, Clifford Simak, Theodore Sturgeon, and many others.
"The landmark series was Dimension X, which was broadcast by NBC....It was the first radio series to treat science fiction in an adult way." (Mike Ashley, Transformations: The History of the Science-Fiction Magazine 1950 to 1970)
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Entertaining short story, short radio program, with some Brownian motion: some twists and turns as Martians and Men mentally fence. The knock on the door changes meaning, from beginning of story to end.