A deadly outbreak of Rigelian fever aboard the Enterprise that sent Kirk, Spock and McCoy to the planet Holberg 917G in search of ryetalyn, where they met the immortal Flint.
In the Original Series episode Requiem for Methuselah, Rigelian fever has killed three of the crew and infected many more, and the whole ship will be lost within hours unless McCoy can obtain ryetalyn, the only known antidote. Sensors find the mineral on a small, supposedly uninhabited planet.
The planet belongs to a reclusive man named Flint, who lives with his ward Rayna and at first orders the landing party away. He relents and has his robot gather and process the ryetalyn, but delays the work while encouraging Kirk and Rayna to spend time together.
Flint is about six thousand years old, and Rayna is an android he built as an immortal companion; he hoped Kirk would wake her emotions. Torn between the two men, she dies. The antidote reaches the ship in time, and Spock quietly eases Kirk's memory of her.