Kirk returns to a planet he surveyed years ago as a younger officer. There he discovers that the villagers and the hill people are now fighting, because the villagers have developed crude firearms and the idea that they should simply take anything they want from the hill people. Worse, Spock is the victim of a gunshot! Kirk and McCoy return to the surface to investigate more completely, but Enterprise must leave to avoid detection - and then a mugato bites Kirk! He must make contact with the hill people, who number among them a kind of witch doctor called a kanutu, who may be able to help. But that help comes with a price...

Historian and sociologist John Gill has stopped responding to transmissions. He is embedded in the Ekosian population to study it. Investigating, Kirk and Spock discover that Ekos has developed nuclear weapons - an achievement well ahead of where they should be scientifically, based on past probes. And they have modeled their society on Nazi Germany, right down to oppression of their sister planet, Zeon. It seems apparent Gill has contaminated this society, but why? And why Nazi Germany?