The opto-aud reveals both sexes of the race from which they are descended, and Theela explains that they came to this planet when their
homeworld began to die. They built the temple and all surrounding it, but they did not know that surface radiation which the planet emitted drained humanoid energy. The women's bodies developed a glandular secretion that enabled them to survive and to manipulate certain areas of the male's brains to influence their emotional senses, ultimately draining the men. This caused them to weaken and die.
To survive the females must revitalize every 27 years. They are eternal prisoners because they neither age nor die, nor can they bear children.
...
Uhura and one of her security officers locate the urn and destroy it with their
phasers, on setting one, releasing the water and the trapped crew members. They are quickly returned to the sickbay of the
Enterprise. Chapel tries everything she knows in her attempt to reverse the aging process, but she fails. Spock suggests that the
transporter is the key to their age restoration, because it holds the molecular pattern of their original bodies when they beamed down. Although feasible, this procedure has never been attempted before. Spock states that the odds are against them, 99.7 to 1. If it fails, their patterns will break up and scatter in space.
Scott, who, despite still having a severe headache he and the other men still all have, is no longer affected by the alien probe. He transports the four aged crewmen to the planet's surface and prepares to transport them back aboard upon reprogramming the transporter. Despite a brief scare, their transport-transformation is successful.
Back in the temple on the planet, Theela destroys the opto-aud, the probe that lured men to their planet, with a phaser, as part of a prearranged agreement. Uhura explains that a crew of women will bring a ship back to transport the alien women to the first suitable planet, where, according to Dr. McCoy, their physiology will return to normal within a few months, ultimately offering them a better future than immortality.