A historian's well-intentioned experiment has turned a society into a copy of Nazi Germany.
The Enterprise travels to Ekos to find a missing historian, John Gill, and discovers that the planet's people have copied the social structure of Nazi Germany. Kirk and Spock disguise themselves and infiltrate the regime, and they learn that Gill's good intentions led him to disaster.
The episode uses a historical setting to examine the dangers of efficiency without morality. Uniforms and propaganda are used as a warning. It shows Kirk and Spock in daring disguise and was controversial, but it is remembered for its serious message about how good ideas can be twisted.