The Parasites' Plot

After losing contact with the planet Deneva (population 1 million) for more than a year, the Enterprise goes to investigate. Uhura's attempts to make contact at all frequencies, including private GSK783 subspace frequency 3, but without success. Spock consults the computer and discovers that waves of mass insanity have swept along a straight path through planetary systems Beta Portalin, Lavinius 5 (200 years ago), Theta Cygni 12, and Ingrahem B (2 years ago). Deneva (colonized a century ago and one of the most beautiful planets in the area) is the next planet on this path, and Kirk expects the worst for his brother Sam and his family.

While approaching the planet, the Enterprise encounters a Denevan ship heading straight for the sun. Attempts to convince the pilot to change course fail, and the pilot flies into the sun. Just before being vaporized, the pilot makes the cryptic statement "I did it. It's finally gone," and claims that he is finally "free." Uhura finally gets through to the private transmitter, and hears the message "Please hurry, help us, I don't have much time. They'll know," being transmitted by Sam's wife Aurelan.

When Kirk beams down with a landing party, a group of armed men attempts to attack them with clubs, while at the same time shouting that they don't want to hurt them. Responding to a scream, the landing party finds Sam (who was a research biologist) dead and his wife Aurelan and son Peter in great pain. Aurelan refers to invaders, but the tricorder shows nothing unusual. After Aurelan and Peter have been beamed to the Enterprise and questioned as far as they can be, Aurelan reports that the invaders came from planet Ingraham B eight months ago after forcing the crew to take them there. She reports that the invaders use pain to control people and are forcing the residents of Deneva to build ships for them.