A doctor at the Tantalus V penal colony whose mind was broken by Tristan Adams's neural neutraliser, and who escaped to warn Starfleet.
Simon van Gelder is a doctor serving as assistant to Tristan Adams at the Tantalus V penal colony. He arrives aboard the Enterprise as a stowaway in a crate, violent and seemingly insane, and is dismissed as a deranged patient by Adams.
Spock's mind meld reveals that van Gelder was tortured with the neural neutraliser, a device Adams uses to empty and reshape the minds of inmates and staff. After Kirk uncovers the truth and Adams dies, van Gelder recovers and is left to restore the colony as a humane institution. He was played by Morgan Woodward.