Star Trek - The Original Series, Vol. 26, Episodes 51 & 52: Return to Tomorrow/ Patterns of Force
"Return to Tomorrow," Ep. 51 - Kirk, Spock, and Dr. Ann Marshall allow noncorporeal beings to inhabit their bodies so that these aliens can prepare androids for themselves. But one entity secretly plans to remain in Spock's body. "Patterns of Force," Ep. 52 - On a routine check of planet Ekos, nuclear missles are fired at the U.S.S. Enterprise. Kirk and Spock investigate and find the planet is controlled by latter-day Nazis.