Guest stars:

David Brian             John Gill, earth historian sent to the planet as a cultural observer responsible for contaminating the civilization of Ekos

Skip Homier             Melakon, deputy feurher
Richard Evans           Isak, young Zayon in jail with Kirk and Spock, instrumental in helping them rectify the contamination of Ekos
Valora Norand           Daras, Nazi party secratary, hero of the fatherland, decorated for betraying her father to the party, actually a spy and defector.
William Wintersole      Abrom, brother of Isak
Patrick Horgan          Eneg, Nazi party chairman, also a defector
Bart LaRue              Newscaster
Ralph Mauer             SS Lieutenant
 Solya, Nazi gunman

Crew Appearances:
Kirk, Spock, McCoy
Uhura
Chekov
Scott
no Sulu
 
On a routine check of planet Ekos, the Enterprise is fired upon by nuclear missiles. Kirk and Spock investigate and find the planet is controlled by 23rd C. Nazis, The Prime Directive has been violated  by a Federation advisor's experiment in efficient government. A  war, rooted in racial hatred, is being waged against a peaceful neighboring planet, Zayon.  Kirk and Spock attempt and succeed in getting the drugged advisor to reverse the campaine to wipe out the Zayons.

Zayon: earthlike, modern civilization of humans with crude interplanetary capability, peaceful

Ekos: earthlike, modern civilization of humans but was fragmented and not organized. Culture was contaminated by a Federation employee a 'few years ago'.

subcutaneous transponder: an unseen device inplanted under the skin by injection to pinpoint location of implantee when unable to use communicator. The Rabindium crystals in the transponder are used by Spock to create a laser beam cutting tool to free Kirk and Spock from jail.

Zayon probe: an unmanned missle carrying a thermoneuclear war head. Spock says that this technology is way ahead of where the Zayons should be.

Vulcan neck pinch used on jail guard
Vulcan neck pinch used on guard outside Gill's room
vulcan Mind Probe used on Gill to enable him to speak and tell his side of the story.

The Non-interference Directive: Federation is not allowed to interfer in the normal course of a planets development in any way.

Quotes:
(He) must be dead." McCoy speculates about John Gill.
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"That helmet covers a multitude of sins" Kirk to Spock.
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"You should make a very convincing Nazi." Spock to Kirk.
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"If we adopt the way of the Nazis, we're as bad as the Nazis." Isak.
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"Very good Mr. Spock. We may make a human of you yet." Kirk.
"I hope not." Spock.