TOS #29: Operation Annihilate

This is a relatively straighforward episode aboult slimy creatures, which are actually a hive mind, taking over the bodies of the people on Deneva and causing them to build ships to spread the creatures to other star systems. The things kill Kirk's brother and infect his nephew.



Spock also gets infected, but is able to control the pain used to manipulate his body and mind. He beams back down to capture one of the nodes of the organism.



The Captain figures out that the thing is sensitive to light, and they test the hypothesis on Spock. Light does remove the creature from him, but also (temporarily, luckily) blinds him. Before realizing Spock's blindness is reversible, this episode created a good-of-the-many/good-of-the-one scenario around Spock which is clearly one of the precursors to the plot of II.