Trivia: The animated series featured the first appearance of the Holodeck (called the Rec Room) in this episode only. Gene Roddenberry wanted to use the Rec Room/Holodeck idea in the original live action series of the 1960s, but found that budgetary constraints were prohibitive at that time. The Rec Room finally appeared once in the animated series, in this episode, in 1973. It wouldn't be seen again until Star Trek: The Next Generation, in 1987, some 14 years later.
Trivia: When the animated series was being voice-cast, the only original actors chosen were William Shatner, Leonard Nimoy, DeForest Kelly, James Doohan and Majel Barrett (Gene Roddenberry's wife). James Doohan was going to double as Chief Engineer Scott and as Lieutenant Sulu, as well as any needed additional male voices; Majel Barrett was going to perform several female voices including the Ship's Computer and Communications Officer Uhura. When Leonard Nimoy (Spock) learned about the casting choices, he threatened to quit the animated series unless original series actors George Takei and Nichelle Nichols were hired to play Sulu and Uhura. Nimoy's opinion carried a lot of weight, so Takei and Nichols were immediately hired.
