The clandestine resistance of Zeons and sympathetic Ekosians who fought the Nazi regime on Ekos and helped Kirk and Spock reach the drugged Führer, John Gill.
The Zeon underground is the resistance movement in the Original Series episode Patterns of Force. The peaceful Zeons had come to the neighbouring planet Ekos to help it develop, only to be made the scapegoats of a Nazi state modelled on old Earth, and marked for extermination.
Kirk and Spock fall in with the movement after escaping a prison cell with Isak, a Zeon captive. Its leader in the capital is Isak's brother Abrom, and its allies include the Ekosian Daras, publicly a decorated heroine of the party, and Eneg, a senior official.
With their help the landing party enters the chancellery posing as a film crew and finds that John Gill is a drugged figurehead controlled by Melakon. Revived, Gill denounces the regime before he is shot.