Space Seed
Kirk discovers a space ship from the 20th century containing a cryogenically frozen, genetically engineered crew. When awakened, these super humans are determined to dominate the galaxy.
It's Khan! His first appearance is a possibly overrated episode, but no doubt he's the best villain of TOS. The alternative-history lesson is fascinating, involving World War III, eugenics experiments, and "supermen" leading up large sections of the world. In the 23rd century it seems a little too easy for Khan's people to seize power. Is it so very easy to take over the Enterprise? One must wonder. Well, they do have help from Lt. McGyvers, an unconvincing character. She lacks true loyalty to either side and has no will of her own. This is distressing in a show that normally places women in a progressive light. The dinner scene is probably best, with the issues of human "improvement" discussed and the underlying tension. Also good is Khan threatening McCoy, who tells his patient precisely how to kill him. Tough stuff. Kirk is saved by McGyvers of all people, and proceeds to beat the five-times-as-strong Khan with a random pole in engineering. Not a very satisfying end. Fortunately the story continues with the rock 'em sock 'em Wrath of Khan.
Spock Saves the Day: Spock is suspicious of Khan from the start, and voices his objections to selective breeding and tyrants.
Oops: Again, it would make more sense to have Khan and Spock duke it out, and leave the average humans out of it. But it's Kirk's duty to fight at each episode's end, even if his victory is not probable or logical.
Developments: Spock doesn't care to speculate on possible benefits of the Eugenics War, seeing only a "dark age." Unlike his follow officers, he is never awed by Khan, who is seen as threatening.