Though most hale "Amok Time", "The Naked Time", "The Man Trap", and "The City on the Edge of Forever", as the best episodes, and maybe they are, but I would have to say that is the most underrated story of all three seasons. It is an over-looked treasure I couldn't wait to get on DVD. The whole story takes place pretty much in one main facility. There are no back-and-forth to the ship or to other planets and stuff, which means the writing had to be extra clever to keep our attention. The big three of Kirk, Spock, and Bones, are forced to show an alien female what it means to sacrifice your life for someone else. The experiment is conducted by two superior alien beings who lose the meaning of life themselves in the midst of all the testing. With all the bickering Spoke and Bones do, you really see how much compassion they have for each other and their captain.