Star Trek: The Original Series/Recap/S1/E01 The Man Trap

Kirk, McCoy and a random crewman beam down to medically examine two scientists, living alone on an archaeology planet. One of these scientists (the lady, Nancy) was apparently previously romantically involved with McCoy. She shows an ability to disguise her appearance, seeming different to each person. She and the random crewman leave and he is killed off screen. Nancy says he ate some of the local vegetation and thus poisoned himself, but any reasonable viewer would doubt that.

McCoy and Spock determine that the unfortunate fellow didn’t die of poisoning and Kirk beams back down to investigate with McCoy and two more random crewmen. Both crewmen are killed, but Nancy disguises herself as one of the dead and beams aboard the ship. Here she proceeds to creep people out as she hunts for salt. It is determined Nancy isn’t on the planet and now Spock and Kirk beam down to question her husband.

Nancy takes McCoy’s form as an alarm sounds for the man she killed on board. Her husband willingly tells them that she’s actually a shapeshifting alien that killed his wife. She’s also the last of her kind – which Kirk finds unimpressive as she is killing his people. They report back to the ship where Nancy continues to impersonate McCoy. They plan to administer a truth serum on her husband so he will reveal where she is... at which point she kills her husband and attacks Spock. She goes to McCoy’s room and tries to convince him not to let them kill her. He is eventually forced to when she tries to kill the captain.

The Fan Nickname for this episode is "The One With... The Salt Vampire".

"Prof. Crater: It's the last one. The buffalo. There is no difference. Kirk: There's one, Professor. Your creature is killing my people!"
 * Bizarre Alien Biology: Spock, obviously. Though why the alien would be uninterested in his blood simply because it was copper based is uncertain. Does he have no salt in his blood?
 * Spock says that he has different blood salts, due to Vulcans evolving from creatures that lived in different oceans.
 * Doppelgänger Replacement Love Interest: Dr Crater's wife Nancy actually died a year or two ago, and the alien has taken her form and lives with him.
 * Fan Nickname: The (unnamed) alien is almost universally known as "the salt vampire".
 * Girl of the Week: And for once, the girl is McCoy’s. And she’s been killed by an alien. Typical.
 * Green Aesop: The episode sounds like it was written to talk about extinction of species. Since this alien happened to be a blood (okay, salt) thirsty alien who was well on their way to mass murder, it was a somewhat Broken Aesop.
 * The broken nature of the Aesop is actually Lampshaded:


 * Last of His Kind: The alien.
 * Red Shirt: Three crewmember deaths in one episode. None of whom were actually wearing red.
 * Science Marches On: It's mentioned in this episode that buffalo are extinct. It is true that overhunting brought the buffalo very close to extinction at the end of the nineteenth century. In the 1960s, it was a fairly reasonable assumption that buffalo might be extinct in the future, although probably not the best guess since conservation efforts had started decades earlier. Today they are no longer considered endangered at all. (The episode also mentions passenger pigeons, which were already extinct when the episode was made.)
 * Shapeshifter Default Form: Though we see the alien’s actual appearance - it looks something like a swamp monster with a fish face and big teeth - it takes this form only when it has been stunned or weakened (and upon its death); it seems to be most comfortable in the form of Nancy, because it is implied that it "feeds" on positive emotions, and both Crater and McCoy are very fond of her.
 * Shapeshifter Guilt Trip: McCoy hesitates to shoot the alien because it has taken the form of his old flame.
 * Shapeshifting
 * This Was His True Form: The alien reverts to its true form after it dies.
 * Truth Serum: Apparently this exists in the future. It will never be brought up again in any episode. Alternatively, it could very well be little more than "truth serum" as it is today - a drug that makes the user highly susceptible to suggestion. Desperate times call for desperate measures.
 * Uncanny Valley: “Nancy” while she was stalking Rand’s salt. Creepy.