“The Immunity Syndrome”
“The Immunity Syndrome” An enormous space-dwelling amoeba is consuming entire planets. The only way to destroy it would be from within. Kirk has to decide whether to send McCoy or Spock on a suicide mission on a shuttle inside the amoeba. He decides that Spock should do it. When the amoeba is about to be destroyed, the shuttle can be pulled out alongside the Enterprise, and Spock is saved. summary by Ex-Astris-Scientia
I must admit something you will rarely hear from me when discussing TOS: Nice effects! Here even I have to appreciate those few shots of the creature.
The whole episode, OTOH, didn’t appeal to me very much. The problem is morality. I so much prefer the TNG attitude towards life! Why, oh, why, don’t they try to contact the creature and reason with it? There’s certainly too much killing in TOS for my taste. I know, I know, different times, different values. Still, I’d rather buy a tricorder than a phaser