Features a rare guest voice in the show with no previous Star Trek (1966) connection: Ted Knight as Carter Winston. Knight goes uncredited here, even though he was famously playing Ted Baxter on The Mary Tyler Moore Show (1970) at the same time.
This is the only episode in which Dr. McCoy's daughter, Joanna, is mentioned. She was to be featured in The Way to Eden (1969), which was originally titled "Joanna," but this idea was dropped.
First appearances of M'Ress, Gabler, and the Romulans in the series. M'Ress is a female Caitian, a catlike species who would make their life-action debut in Star Trek IV: The Voyage Home (1986), where one appears as a Starfleet admiral attending James T. Kirk's court martial, and another as a commodore.
Uhura appears but does not have any lines as Nichelle Nichols was also voicing Anne Nored. Kirk gives his orders to new character M'Ress instead.
Features one of the first truly exotic non-humanoid aliens ever depicted in Star Trek. The shape-shifting "Vendorian" would not have been possible in the live Trek show, as the special effects technology necessary didn't yet exist. The "Organians" in Errand of Mercy (1967) and the "Medusan" in Is There in Truth No Beauty? (1968) were simply light effects.