#50: The Man Trap
In case you forgot: A shape-shifting salt vampire, whom Bones believes is his beloved ex-girlfriend, chows down on some redshirts. She spares McCoy because he has such strong feelings for her (she also feeds on love...?), but turns on her husband and the captain. McCoy is the one who has to shoot her to save Kirk and the ship, of course, because of the Rule of Drama.
Getting to see the rest of the crew of the Enterprise have some more lines and some more personality is a nice change of pace. This could be either a good or bad thing, depending on how you feel about the somewhat painful exchange between Spock and Uhura; personally, I wish it had carried over into later episodes (though written more competently). Regardless, a salt vampire is kind of a cool idea for a monster, shape-shifting or otherwise, and the pathos of the species' extinction at the hands of the good doctor doesn't feel at all hamfisted.
Also, I'm probably the only person in the world who thinks the handpuppet flower in Sulu's botany laboratory is a cool, cute effect.
But what potential this episode had is squandered pretty quickly. (There's a reason it's not ranked higher, after all.) McCoy may get a backstory, but what he gains in character development he loses in smarts. We all do stupid things for love, but Bones makes some decisions that are just, well, bone-headed. (Hah, hah.) To be fair, McCoy isn't the only one holding the idiot ball: Kirk and the crew seem to be pretty unnaturally slow on the uptake that they're dealing with a shape-shifter.
There's also far more shots of the creature wandering around the Enterprise looking for a meal than is really necessary. Smells like filler, to be honest.