Episode Behind the Scenes

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First instance of a "parallel Earth." Others would appear in "The Return of the Archons", "Bread and Circuses", "A Piece of the Action", "Patterns of Force", and "The Omega Glory".
Final appearance of Lieutenant John Farrell, now replacing Uhura at the communications station.
The little girl that Captain Kirk picks up when confronting the children is William Shatner's daughter, Melanie. She would later portray the role of the Captain's Yeoman in "Star Trek V: The Final Frontier".