The U.S.S. Enterprise is investigating Beta III, where the U.S.S. Archon disappeared over 100 years before. When the landing party exhibits strange behavior, Kirk sends another party down to investigate. They find the culture on Beta III is quiescent, with no creative tendencies. The entire culture, known as "The Body", is controlled by a group of lawgivers, who are in turn controlled by the omniscient Landru.

Harry Townes (Reger)
Torin Thatcher (Marplon)
DeForest Kelley (Dr. Leonard H. McCoy)
Brioni Farrell (Tula)
Sid Haig (First Lawgiver)
Charles Macaulay (Landru)
Jon Lormer (Tamar)
Morgan Farley (Hacom)
Christopher Held (Ensign Lindstrom)
George Takei (Lt. Hikaru Sulu)
James Doohan (Lt. Cmdr. Montgomery Scott)
Nichelle Nichols (Lt. Uhura)
Sean Morgan (Lt. O'Neil)
Ralph Maurer (Bilar)
David L. Ross (Lt. Galloway)
Eddie Paskey (Lt. Lesley)

• In one of Star Trek: The Original Series' recurring themes, the Beta III society is controlled by a computer, Landru. The computer was created by the real Landru 6000 years ago. Kirk convinces Landru that it is evil and it destroys itself.
  
• Sociologist Lindstrom remains behind with a party of experts to help the Betans rebuild their culture.
  
• Spock apparently is able to sleep with his eyes open.