Just over a month before this episode aired, guest star Ricardo Montalban had played the character Khan Noonian Singh for the first time in Space Seed (1967), and here worked with Mr. Sulu himself, George Takei, who had not appeared in that "Trek" episode. The two would be castmates again 16 years later in Star Trek II: The Wrath of Khan (1982), but in that film the only "Trek" regular acting on the same soundstage as Montalban would be Walter Koenig, who also did not appear in "Space Seed," debuting his Mr. Chekov character in the show's following season.