In 2266, he was assigned to the Tantalus Penal Colony as Dr. Tristan Adams' associate. He was later promoted to director. By the time the USS Enterprise visited, six months later, he was suffering from the effects of neuro-synapse damage.
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 a neural neutralizer, an experimental therapeutic device. 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 at the colony, 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")
Background information Edit
In the final draft script of "Dagger of the Mind", Van Gelder was initially described as "the large wild figure of a man [...] with some of the watchfulness of a hunted animal" and a "tall, unkempt, desperate looking man. In time, we'll come to recognize much of this as a refreshing throwback to 19th century individualism with all of its exasperating foibles, groping nobility of spirit. But all we're aware of at this moment is his violent emotional state, the fact he is obviously dangerous."
Also in the script, Van Gelder was to have suffered medical complications in the aftermath of his mind meld with Spock. The stage directions in the script stated, "McCoy has glanced up at the [body function] panel, finding the readings dropping alarmingly [....] Van Gelder's head has fallen limply to one side. McCoy moves in fast, slapping Van Gelder across one cheek, then another [....] McCoy quickly scrambles through a medic kit, finds a small white pill, inserts it into Van Gelder's mouth. A long wait, then Van Gelder begins to slowly breathe again, his eyes flutter, he drifts back toward life."
In playing Van Gelder, Morgan Woodward aimed to build the character to a frenzied state but keep him realistic. Regarding how this performance was received during the making of "Dagger of the Mind", Woodward noted, "Everyone [on the set] was supportive and admiring of my work." (Starlog, issue #130, p. 72)
The role of Van Gelder was so emotionally expressive that, reportedly, Morgan Woodward, after playing the part, had to rest at home for four days.  Woodward himself reflected, "After the episode had been shot, I went home and closed the door. I stayed inside my house for the next four days just recovering. I was beat. I was so sore, physically, from fighting that straitjacket in the Sick Bay scenes, and all I wanted to do was rest. All of that screaming, crying and yelling exhausted me, physically and emotionally." Comparing Van Gelder to Woodward's only other Star Trek role – Captain Ronald Tracey in TOS: "The Omega Glory" – the actor remarked, "Van Gelder was much more difficult because it required tremendous amounts of emotional energy." (Starlog, issue #130, p. 72)
When Morgan Woodward first watched "Dagger of the Mind", seeing his part in the show was a crushing blow to him. "I couldn't believe it. I was very disappointed in my performance. I thought it was very overdone. I felt the director [Vincent McEveety] had just abandoned me and let me go crazy, so to speak. But then, I had people coming up to me and congratulating me on my performance [....] If the role was terribly overdone, which I initially felt it was, it was at least within the realm of science fiction, where almost anything is permissible." (Starlog, issue #130, p. 72)
Interviewed in 1988, Morgan Woodward further commented, "Van Gelder is the most physically and emotionally demanding role that I have ever played. That is a role I will never forget. It is still vivid in my memory [....] Just seeing 'Dagger of the Mind' again exhausts me!" (Starlog, issue #130, p. 72)
Dr. Simon Van Gelder's name was mentioned on a Planet 10 Shipping label, which is visible in DS9: "A Simple Investigation" (though with the small print, including his name, blurred) and in the Star Trek Encyclopedia (3rd ed., p. 448).