Dr. Simon Van Gelder was assigned to the Tantalus Penal Colony in early 2266, as Dr. Tristan Adams' associate. By the time the USS Enterprise visited, six months later, he was severely mentally ill, apparently suffering from delusions, paranoia, and partial retrograde amnesia.
Van Gelder escaped the colony by concealing himself in an outbound cargo container, then confronted the bridge crew before he was captured and taken to sickbay. Dr. Adams told Captain Kirk that Van Gelder had injured himself with an experimental therapeutic device, the neural neutralizer. According to Adams, Van Gelder felt he hadn't the moral right to expose another man to something he hadn't tried on himself first.
Van Gelder had great difficulty remembering basic facts, such as his name, his duties, and who he worked with. Each of his attempts to remember was enormously painful. He was violently agitated, and desperate not to return to the penal colony. Although Dr. Adams offered reasonable explanations for all of this, Dr. McCoy was suspicious, for reasons he couldn't clearly explain. Regulations required Kirk to answer McCoy's doubts, and so Kirk visited the colony with psychiatrist Helen Noel to investigate Van Gelder's accident.
Van Gelder consistently maintained that he was not a criminal, and did not require the neural neutralizer. When he learned Kirk planned to spend the night on the surface, he became agitated again, and warned Spock and McCoy that Kirk was in terrible danger, and that Dr. Adams would destroy him. Because of his profound symptoms, it was difficult for the officers to understand or believe him.
To learn the truth, Spock was forced to attempt something he had never tried with a Human – a Vulcan mind meld. Despite the risks, Van Gelder insisted Spock make the attempt. Spock learned that Dr. Adams had deliberately used the neural neutralizer on Van Gelder, reshaping his thoughts and emptying Van Gelder's mind so that any thought Adams uttered would become Van Gelder's thought.
Van Gelder's revelations prepared Spock to act, so that when Dr. Noel temporarily interrupted the power, Enterprise personnel were able to secure the colony. Dr. Van Gelder was restored to health, and resumed his responsibilities. He also dismantled the neural neutralizer room and destroyed the equipment. (TOS: "Dagger of the Mind")