During a deadly epidemic, Kirk meets an immortal recluse named Flint on a remote planet and falls in love with Rayna, the young woman in his care.
An outbreak of Rigellian fever aboard the Enterprise sends Kirk, Spock and McCoy to the planet Holberg 917G in search of a mineral needed for the cure. There they meet Flint, a recluse who owns art and music by great figures of Earth history, along with his robot servant and his ward Rayna.
Flint turns out to be an immortal who has lived for thousands of years under many names, including some of those famous artists. Rayna is an android he built to be his companion, and when Kirk falls for her she is torn between the two men and dies under the emotional strain. Afterward Spock uses a mind meld to help the grieving Kirk forget.