 The Enterprise tracks the missing Federation s cultural observer professor John Gill to the reportedly primitive, peaceful planet Acos; when Kirk and Spock beam down, they find the Acosians have turned into a Nazi society at war with the Zeon people, with Gill as its Führer. They steal uniforms to enter the headquarters but when Spock is forced to remove his helmet his ears betray them, straight to the torture chamber. After a flogging as  Zeon spies  they manage to escape and take with them the Zeon prisoner Eneg who brings them to the Zeon resistance, which tests them and plot with an Acosian defector to get to Gill, posing as a Nazi propaganda film crew. Inside they discover things are not quite as they appear...