The Enterprise travels to Exo III in search of exobiologist Dr. Korby, fiancé of Nurse Chapel. Using equipment left behind by "The Old Ones," the extinct natives of Exo III, Korby has constructed androids who look and act like humans and he intends to replace key people in the Federation with androids to integrate the machines into other worlds — beginning with Captain Kirk.

Michael Strong (Dr. Roger Korby)
Sherry Jackson (Andrea)
Ted Cassidy (Ruk)
Majel Barrett (Nurse Christine Chapel)
Nichelle Nichols (Lt. Uhura)
Harry Basch (Dr. Brown)
Vince Deadrick (Crewman Mathews)
Budd Albright (Crewman Rayburn)

• Korby has been missing for 5 years. Christine Chapel gave up a promising career in biology to serve on a starship that might one day find Korby. Chapel decides to stay with the ship at the end of the episode.
  
• Ruk is the last remaining android built by the Old Ones. The Old Ones became afraid of their machines and began to deactivate them. The machines overcame their programming and killed their inferior creators.