Ensign Berkeley
Larry Anthony

Dr. Tristan Adams
James Gregory

Dr. Simon Van Gelder
Morgan Woodward

Dr. Helen Noel
Marianne Hill

Lethe
Suzanne Wilson

Having left the Tantalus V penal colony after a routine visit, a stowaway is discovered aboard the Enterprise which turns out to be Dr. Simon Van Gelder, a former director of the colony. Kirk contacts the current director, Dr. Adams, and is informed that a failed experiment was responsible for Van Gelder's insanity.

The Enterprise returns to the planet and Kirk beams down with Dr. Helen Noel to examine the facility. They are given a tour and shown the machine which caused Van Gelder's condition - a 'neural neutraliser' - that, they are told, is now functional but only capable of relaxing agitated inmates for short periods of time.

Kirk is sceptical and, during the night, he and Dr. Noel secretly test the machine. They discover that it is much more powerful than claimed when, even on a low setting, Kirk finds he is compelled to follow suggestions made to him by Helen Noel, who is operating the device. They are discovered there and Noel is captured by Adams, leaving Kirk at his mercy, strapped in the neural neutraliser. Adams plants the suggestion in Kirk's mind that he is madly in love with Helen, following which he becomes absolutely besotted with her.

They are unable to contact, or be rescued by, the Enterprise crew due to a security force field that has been thrown up around the facility. Dr. Noel is finally able to convince Kirk of their duty and he allows her to leave him and shut down the power supply generating the force field. During the period of their captivity Spock had been busy aboard the Enterprise. Mind-melding with Dr. Van Gelder, he has learned that the doctor had been brain-washed by Adams but has been unable to contact the captain to advise him of the fact until Noel shuts down the force field.

While Helen Noel is on her sabotage mission, Kirk fights with Adams, knocking him unconscious and leaving him in the chair of the neural neutraliser while he escapes. When power is restored to the colony the machine activates at full strength. With no-one at the controls to place suggestions to him, Adams' mind empties completely, causing his death.