The U.S.S. Enterprise is on a routine mission in deep space when a beautiful young woman beams onto the bridge, touches a band on her wrist, rendering everyone unconscious. When the crew awakens, McCoy summons Kirk to sickbay and informs him that the woman has removed Spock's brain and if it isn't returned to his body within 24 hours, Spock will die. The Enterprise pursues the woman's ship to the Sigma Draconis IV system. When Kirk and party beam down, they find a rough, frozen world inhabited by two peoples: the Morgs, primitive men who live on the surface, and the Eymorgs, females who live deep underground in a comfortable, highly technological city, yet have primitive, child-like minds. Kirk finds Spock's brain, which is now attached to the Eymorg central control system and is now revered as the "Controller."

William Shatner (Captain James T. Kirk)
Leonard Nimoy (Commander Spock)
DeForest Kelley (Dr. Leonard H. McCoy)

James Doohan (Lt. Cmdr. Montgomery Scott)
Marj Dusay (Kara)
Walter Koenig (Ensign Pavel Chekov)
George Takei (Lt. Hikaru Sulu)
Nichelle Nichols (Lt. Uhura)
Majel Barrett (Nurse Christine Chapel)
James Daris (Morg)
Sheila Leighton (Luma)

• NBC moved Star Trek to Fridays at 10:00 PM.
  
• Lee Cronin is a pseudonym of former writer/producer Gene L. Coon; presumably he didn't want his name attached to this bad episode.
  
• In a recurring theme, Kirk destroys yet another civilization run by a machine.