Enterprise stops at the Tantalus penal colony to deliver drugs and collect research materials. Except that the research materials box contains a dangerous madman, Simon van Gelder - the former director of the colony. Kirk and psychiatrist Helen Noel beam down to speak with Tristan Adams, to learn what happened to van Gelder. It seems Dr. van Gelder was the victim of a new device, a neural neutralizer. But was what happened really an accident, or something more sinister?

Spock diverts the Enterprise to Starbase 11, where Kirk discovers that Spock's former commander, Christopher Pike, has been seriously injured and disabled by exposure to delta rays, experienced while saving a class of cadets. Pike is now confined to an elaborate life support apparatus. Spock abducts Pike and locks the Enterprise on a course for Talos IV, the only planet subject to General Order 7: visiting the world is a capital crime. Spock refuses to explain his actions, but demands a court martial, during which he presents evidence of an earlier visit. War, thousands of centuries ago, devastated the planet, and it was only now becoming able to support life again. Its inhabitants, forced underground, had developed enormous mental powers, including the ability to place illusions within the minds of others. Their plan was to breed a race of slaves to help them reclaim their world, as they were fragile and their civilization degenerate: illusions had proved a kind of narcotic. To this end, they kidnapped Captain Pike years ago...