A shape-shifting, salt-craving creature terrorizes the crew of Enterprise.

William Shatner (Captain James T. Kirk)
Leonard Nimoy (Lt. Commander Spock)

Jeanne Bal (Nancy Crater)
Alfred Ryder (Prof. Robert Crater)
DeForest Kelley (Dr. Leonard H. McCoy)
Grace Lee Whitney (Yeoman Janice Rand)
George Takei (Lt. Hikaru Sulu)
Nichelle Nichols (Lt. Uhura)
Bruce Watson (Crewman Darnell)
Michael Zaslow (Crewman Green)
Vince Howard (Crewman)
Francine Pyne (Nancy III)
Eddie Paskey (Lt. Lesley)

• It seemed really unnecessary to kill the pitiful creature.
  
• After the creature boards the Enterprise in Green disguise, he sees Yeoman Rand carrying a lunch tray, and nibbling from it; when "Green" reaches for the salt, she slaps him away with a "Who do you think you are?!" She continues to the bio-lab and then gives the tray to Sulu! He should have slapped her hand for eating from his plate.
  
• This is the first episode McCoy speaks the infamous "He's dead, Jim" — referring to Crewman Darnell, the first crewman to die under Kirk's command of the Enterprise.
  
• Spock is the only character to appear in both the unaired pilot ("The Cage") and the series.
  
• This is the first appearance of Dr. McCoy, Lt. Sulu, Lt. Uhura, and Yeoman Rand.
  
• This is the first appearance of extra Lt. Lesley, who is uncredited in this episode and doesn't receive a name until "The Alternative Factor".
  
• Only William Shatner and Leonard Nimoy are listed as the main cast in the opening credits. James Doohan, George Takei, Nichelle Nichols (and Walter Koenig, Season 2+) were listed as co-stars in the end-credits throughout the series. DeForest Kelley is listed in the end-credits in this season, but becomes a regular cast member in Season 2.