An escaped inmate from a penal colony reveals that its director is experimenting on his patients.
After a man escapes from the Tantalus penal colony in a cargo container, the Enterprise visits to investigate. Kirk meets Dr. Tristan Adams, who runs the colony and uses a device called the neural neutralizer. Kirk and a psychiatrist, Helen Noel, soon realise the machine is being misused.
The episode introduces the Vulcan mind meld, which Spock uses on a character for the first time. It is a commentary on the dangers of experimental treatment of prisoners, and the story is often remembered for the unsettling use of a machine that can alter the mind.