Appearances can be deceiving.
McCoy sees Nancy Crater as he wants her, like she looked ten years earlier – not what’s actually there. Kirk looks at the same girl and sees something totally different – he sees her as she should look now. The red shirt sees something different still. All three guys look at the same figure and each sees something different.
By attracting people to itself, the salt vampire can reveal its true nature and devour its prey(‘s salt). This is a classic concept, in no way exclusive to Christianity. The Sirens tried to lure Ulysses and his men to a rocky death. These tired, stinky sailors had spent the previous decade in war. Beautiful feminine voices naturally caught their attention. The Christian analogue is the concept of temptation.
So Nancy Crater / Sirens / Temptation ->> Salt Vampire / Rocky death / Sin.
All of the temptations are appealing, because who would be attracted to death?
And we try to look only at the pretty stuff, ignoring the dangers that may lie on the other side of it. Certainly not all pretty stuff leads to death. But we usually know the difference. Does the given activity have to be defended with a “better-to-burn-out-than-fade-away” attitude? Then it’s not pure beauty; it’s likely temptation leading to death. Does it just feel right, despite the fact that you know it’s wrong? It’s temptation leading to death. Sleeping around, stealing pretty things, anything that involves breaking the law, get-rich-quicks. These are probably not times we’re enjoying pure beauty or love or anything. These are most likely temptations that will appeal to our base desires and encourage us to sin. Just like the Sirens and Nancy Crater, we can’t just have convenient sex. Neither can we have convenient wealth or comfort. Usually. Temptation appears as a short cut to joy, but is really just an on-ramp to … the Highway to Hell! (I just couldn’t resist!)
We try to inoculate sin by focusing on the appealing temptation side of it. We ignore the syphilitic results of indulgence. We ignore the heart disease that follows comfy laziness and delicious third helpings. Most people only see the words ‘sin’ or ‘temptation’ on dessert menus.
Stories like this remind us that the joy we find in the temptation to do evil is brief. It’ll suck the life outta ya!
The whole thing wraps tightly around McCoy (in his very first ever appearance on TV, by the way) struggling with the knowledge that all evidence points to this not being Nancy and yet wanting it so badly to be her anyway. The creature has killed and left its saltless victims strewn about the planet and the Enterprise. It creeps closer and closer to McCoy’s quarters. He’s face to face with the woman he wants and he knows it’s not her, but he just can’t bring himself to fire his phaser.
"Get behind me, Satan!" Um...that's not quite what that verse means, Doc.
Spock even rushes in to try to overpower the thing but fails. If McCoy refuses to see the truth, there’s no saving him. Finally, Spock hollers out, “Is that Nancy, Doctor?”
What are we tempted by? Whatever it is, we have two ways to get free from it. Our friends can just tie us to the mast so we’re not physically able to get to the sirens, left to wonder what might have been had we been allowed to indulge. Or someone can challenge us to face the truth.
“Is that really Nancy?”
“Is that affair really going to restore your confidence?”
“Is that double bacon quarter-pounder that’s oozing all over your fingers really going to reduce your stress from work?”
“Are you realy more important than the other people in line?”
“Will making that much more finally be enough for you to start giving to others? Or will the high cost of living discourage you, no matter how many raises or lucky breaks or freebies you get?”
“Is screaming at your wife really going to make her finally see things the right way?”
“Will dreaming about having that guy’s house and wife get you any closer to having your own house? Or finding someone to marry?”
“Is it really demonstrating your maturity by rejecting all correction?”