At the Tantalus V penal colony, Dr. Tristan Adams secretly used a device called the neural neutralizer to empty and reshape the minds of inmates and staff.
The Tantalus V penal colony had a reputation as a model of humane treatment under its director, Dr. Tristan Adams. When an escaped patient, Dr. Simon van Gelder, reached the Enterprise in a disturbed state, Kirk and psychiatrist Dr. Helen Noel went to the colony to investigate. They found that Adams was using an experimental neural neutralizer to wipe minds and implant his own suggestions.
Adams used the device on Kirk, making him believe he loved Noel. Spock learned the truth through a mind-meld with van Gelder, the first such meld shown on screen. Noel shut down the colony's power, and Adams was left alone in the machine and died. The device was then dismantled.