In 2268, Starnes, his wife, and the rest of his party were all poisoned to death by self-administered cyalodin, while under the influence of Gorgan, a malevolent entity, the last of its kind, found surviving in a cave on the planet. He lived long enough to briefly encounter the USS Enterprise landing party which responded to their distress call. Starnes was then buried on the planet by members of the landing party, along with the other casualties of the expedition.
Prior to his death, Starnes had recorded several logs which detailed his growing sense of anxiety, something he initially attributed to the typical nervousness upon beginning a new project, but which afflicted other members of the party as well. He requested that Starfleet Command send a starship to transport something, but was unable to remember what he was supposed to be sending. This led him to the conclusion that his actions and reactions were being influenced by Gorgan, and decided to send a dispatch to Starfleet, warning them of the threat. Unfortunately, the dispatch was never sent.
Upon viewing the logs, Spock opined that the party must have been overwhelmed extremely quickly, as Starnes would have given more details regarding the expedition's takeover by the entity if he had had the time.
While questioning Tommy about his father's behavior, James T. Kirk asked whether the professor seemed upset. Tommy replied that he was, but that he didn't ask his father why. When Kirk pressed Tommy for details on the reasons for the professor's mood, Tommy told him that his father was always upset, just like Kirk.