Executive producer: Gene Roddenberry
Associate producers: Edward K. Milkis and Gregg Peters
Producer Fred Freiberger
Co- producer: Robert H. Justman
Story consultant: Arthur H. Singer

Writer: Lee Cronin (Gene L. Coon)
Dir.: Marc Daniels

Captain Kirk: William Shatner
Science Officer, First Officer, Cmdr. Spock: Leonard Nimoy
Dr. Leonard "Bones" Mc Coy: DeForest Kelly
Lt. Sulu: George Takei
Lieutenant Commander Montgomery Scott: James Doohan
Lieutenant Uhura: Nichelle Nichols
Nurse Chapel: Majel Barret
Kara: Marj Dusai
Luma: Sheila Leighton

Stardate 5431.4

A young woman suddenly appears on the bridge and everybody is rendered unconscious. When they wake up they find Spock's brain missing. They follow a trail which leads the Enterprise to the 6th planet of the Sigma Draconis system. There the Morgs (man) live on the planet' surface in primitive conditions and the females (Eymorgs) live inside the concrete mountains where they are provided with a system which is beyond their comprehension. 

Only with the help of a device that Kara, the priestess, puts on her head (called: "The Teacher"), she becomes temporarily able to perform the operation to fetch a new brain when the old ones is used up – a brain which maintains all their systems to maintain life in the highly advanced caves. To prevent the landing party to take Spock's brain back, the Eymorgs force them to wear a belt that cause to the wearers terrible pain when a special device is activated. Only Spock, deprived of his brain, can't be affected. Scott manages to guide the body to neutralizes Kara and her device. With the help of "The Teacher" McCoy reverses the operation and from now on the Morgs and Eymorgs are encouraged to live together, helping themselves without the help of a system run by a brain.