The U.S.S. Enterprise is ordered to pick up Ambassador Robert Fox, who is headed to planet Eminiar VII on a diplomatic mission. Upon arriving at the planet, the ship is warned away because Eminiar is at war with Vendikar, the third planet in the Eminiar system. Beaming to the surface with a landing party, Kirk discovers that the war between the two planets is controlled by computers, which determine the damage done and the fatalities caused by the attacks, and assigns citizens to report to disintegration machines, which they do so willingly.

David Opatoshu (First Councilman Anan 7)
Gene Lyons (Ambassador Robert Fox)
DeForest Kelley (Dr. Leonard H. McCoy)
James Doohan (Lt. Cmdr. Montgomery Scott)
Barbara Babcock (Mea 3)
Miko Mayama (Yeoman Tamura)
David L. Ross (Lt. Galloway)
Nichelle Nichols (Lt. Uhura)
Sean Kenney (Lt. DePaul)
Robert Sampson (Sar 6)

• The last starship to visit Eminiar VII was the U.S.S. Valiant, over 50 years ago. The ship never returned from the mission.
  
• The United Federation of Planets is mentioned for the first time in this episode.
  
• Spock mentions for the first time that "Vulcanians" have telepathic ability, which he uses to control an Eminiaran guard, forcing him to open their locked door. (This seems to contradict the episode "Where No Man Has Gone Before", where Mitchell and Dehner have the highest ESP ratings on the ship).
  
• Ambassador Fox stays behind to mediate peace between Eminiar and Vendikar.
  
• Kirk gives "General Order 24" to Scotty — the destruction of the planet's inhabitable surface.
  
• The Eminiar disrupters make a loud noise, but no visible beam.