Kirk and Dr. Helen Noel, the ship's psychiatrist, tour the Tantalus Penal Colony as the Enterprise delivers supplies. When Dr. Simon van Gelder of the psychiatric staff escapes to the U.S.S. Enterprise exhibiting signs of manic insanity, an enraged McCoy insists that Kirk investigate the colony. They discover that the Colony director, Dr. Tristan Adams, has been using a brainwashing device, the neural neutralizer, to control not only the colony's inmates, but his staff as well.

DeForest Kelley (Dr. Leonard H. McCoy)
James Gregory (Dr. Tristan Adams)
Morgan Woodward (Dr. Simon van Gelder)
Marianna Hill (Dr. Helen Noel)
Nichelle Nichols (Lt. Uhura)
Susanne Wasson (Lethe)
John Arndt (First Crewman)
Larry Anthony (Ensign Berkeley)
Ed McCready (Inmate)
Eli Behar (Therapist)

• This is the first appearance of the Vulcan mind meld.