After the Enterprise travels to the planet Exo III in order to investigate the fate of Roger Corby (Nurse Chapel's former fiancee), security guards Matthews and Rayburn are both killed after beaming down with Kirk and Nurse Chapel. In turns out that Corby, known as the Pasteur of archeological medicine, has discovered the remains of an ancient culture. Using machinery he has found, he creates androids Braun and Andrea with the help of Rok, a still-functioning android from the ancient days of Exo III.
Corby begins implementing a dastardly plane by creating an android of Kirk to be taken to Minas 5 where he will start spreading androids throughout the galaxy. The real Kirk and Corby's android are completely indistinguishable. The android even knows that Kirk has a brother named George Samuel Kirk (called ``Sam'' by Kirk only) who saw Kirk off with his wife and three sons.
Fortunately, Kirk had shown great presence of mind while his mental patterns were being duplicated. By concentrating on the words ``Mind your own business, Mr. Spock. I'm sick of your half-breed interference! Do you hear!?,'' he engrained this phrase into the circuitry of the android, who dutifully repeated them when aboard the Enterprise. Hearing these words from Kirk, Spock suspects something is amiss and follows the android down to investigate.
In the meantime, Corby has been forced to destroy Rok, who has remembered the equation ``existence, survival must cancel out programming.'' This equation led Rok to realize that the same kind of clash between humans and androids which led to his civilization's demise centuries ago is becoming inevitable again, and caused him to attempt to kill Corby. However, Corby catches his hand in a door, revealing the fact that he himself is an android. Andrea then destroys the Kirk android when he refuses to kiss her. When she tries to kiss Corby, Corby destroys them both, ridding the universe of Exo III androids for all times.