Stardate 2715.1: While transferring cargo to the Tantalus Penal Colony, one of the inmates beams aboard the Enterprise and manages to make his way to the bridge before being disabled by a nerve pinch from Spock. By the time the Enterprise returns to Tantalus, McCoy has run a few tests on their stowaway and urges Kirk to begin an investigation of the penal colony despite the reputation of its director, Dr. Tristan Adams. Kirk beams down with Dr. Helen Noel, a psychiatrist, to learn more about the colony's activities. Meanwhile, Spock and McCoy discover that their patient is Dr. Simon van Gelder who was, six months ago, a part of the staff at Tantalus. Eventually, Kirk and Noel learn about a device, the neural neutralizer, that Adams is using to brainwash his patients. While testing the device, Adams takes the controls and uses the pain of the neutralizer's beam to force Kirk into surrendering his phaser and communicator. Because of Dr. van Gelder's conditioning under the device, he is unable to fully explain what has been happening on Tantalus. Spock, therefore, decides to risk a mind meld with van Gelder in which he learns the entire truth. Meanwhile, on Tantalus, Dr. Noel escapes via air-conditioning duct from the room where she is held and finds the power room. After a brief struggle with a guard, she lowers the defensive shields which allows Spock to beam down with a security detail. Kirk, who had been under the neutralizer's beam when the power died, is able to fight and get away from Adams who is left on the floor of the treatment room. When Spock restores power the neutralizer engages and drains the mind of Adams who dies from loneliness while under the beam. Dr. van Gelder is returned to Tantalus as the director of treatment. He dismantles and destroys the neural neutralizer.