The 64th episode of Gene Roddenberry’s science fiction series Star Trek airs on NBC. This marks the first appearance of the Tholians and, though they will be mentioned frequently in later Star Trek spinoffs, the last appearance until the 21st century spinoff series Star Trek: Enterprise.
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Also of interest...
Tales Of The Unexpected: A Picture Of A Place
August 16, 1980
Star Trek: TNG: Who Watches The Watchers
October 16, 1989
The Tomorrow People: The Origin Story – Part 3
December 2, 1992