Kartan
Dave Armstrong

Peter Kirk
Craig Hundley

Aurelan Kirk
Joan Swift

Yeoman Zahra Jamal
Mauriska

Tracking the course of a 'plague' that turns its victims insane, the Enterprise heads towards the planet Deneva where Kirk's brother Sam lives with his wife Aurelan and son Peter. Approaching the planet they observe a Denevan ship which ignores their attempts at communication and flies straight into the sun. Just before its destruction the pilot inexplicably transmits the words 'I'm free!'

A landing party beams down to a strangely quiet city and Kirk heads for Sam's laboratory, coming under attack from some insane inhabitants en route. On getting there he finds that his brother is already dead and that Aurelan and Peter are already 'infected'. They are beamed up to the Enterprise sickbay but, after explaining that alien parasites came to the planet eight months previously and forced the inhabitants to build ships for them, she dies in agony.

One of the parasites is also entwined in Peter's central nervous system. The landing party beams back down and locates a nest of the parasites, which resemble flying jellyfish, inside one of the buildings. While trying to capture one for McCoy to examine, another one attaches itself to Spock's back, infiltrating his nervous system before it can be ripped off. The creatures control their victims through pain, but the Vulcan's mind control manages to cope with it while he obtains a specimen and returns to the ship.

Despite many attempts, McCoy and Spock fail to find an effective way of removing the parasite. Kirk remembers the dying words of the Denevan pilot they encountered earlier which leads them to experiment with intense light and succeeding in killing the sample parasite. Spock is exposed to a powerful blast of the full spectrum which also succeeds in killing his parasite but which also leaves him completely blind. Further experiments show that they only needed to have used ultraviolet light and that he lost his sight needlessly.

The Enterprise is geared up to bombard the planet with massive doses of UV light which kills of all the remaining parasites, leaving the remaining inhabitants unharmed. Later, as Kirk is on the bridge recording his report, Spock walks in and continues his duties as if nothing has happened. On asking what is going on, Kirk is told that, due to the intense brightness of Vulcan's sun, Vulcans have developed a kind of 'inner eyelid' which automatically reacted when Spock was exposed to the bright light and that the resulting blindness had proved to be only temporary.