#53: Space Seed
In case you forgot: The Enterprise encounters an old Earth vessel from 200 years ago, with everyone aboard in suspended animation. Turns out they're all war criminals (including KHAAAAAAAN), and they take over the ship with the intent to find a planet to rule. Their plans are foiled and they are instead sent to an uncolonized planet as punishment.
Oh yeah, nerds. I'm going there. Space Seed is one of the worst Trek episodes of all time.
My biggest grief is Marla McGivers. She sucks. When she initially refuses to help Khan take over the ship and he crushes her hand (remember, he's five times stronger than your average dude), it's just...uncomfortable. Pair that with the doe-eyed devotion Marla exhibits for him despite all that and it's too much like actual real-life abusive relationships to be able to watch.
Even if Marla is my biggest grief, she's hardly my only one. The chuckling and grudging admiration Kirk, Bones, and Scotty all have for Khan leaves another sour note over the whole episode; I come down squarely on Spock's side in this case. Imagine if someone found a cryogenically frozen Hitler and woke him up 200 years after World War II, and, after learning about the atrocities he committed, just laughed them off because he had been such a great leader of men. Then when Hitler tries to take over their starship and kill their crew, they just laugh some more and decide that rather than submit him to the justice system to which he rightfully belongs they'll just do him a favor and give him a whole freaking planet.
What the fuck, guys?
The only good thing to come out of this is Wrath of Khan, which is a near-flawless movie that may or may not have had me bawling like a small child. And for those of you complaining about continuity re: Chekov and "I never forget a face" Khan: