Rigellian fever, an extremely deadly plague, strikes the U.S.S. Enterprise crew. Kirk, Spock and McCoy beam down to a supposedly uninhabited planet, Holberg 917-G, in search of the only known antidote, ryetalyn. They encounter a man named Flint, who is not pleased to have visitors. Flint orders his M4 robot to gather and process the ryetalyn, while the three officers are entertained by his lovely companion Rayna, who responds to Kirk's romantic pursuit towards her. Spock soon discovers Flint is an immortal, who wandered the Earth for centuries in various personas, including Brahms and da Vinci. He came to this planet to retire in peace and built the "Rayna" android as his companion.

James Doohan (Lt. Cmdr. Montgomery Scott)
James Daly (Flint)
Louise Sorel (Rayna Kapec)
Nichelle Nichols (Lt. Uhura)