A salt-hungry "vampyre" stalks the Enterprise, able to change shape and assume alternate identities.
There are things that are fun in this episode, shape-shifters lend a lot of possiblities. My favorite part is that there could be a recreational "Ripley's Pleasure Planet" in the 23rd century - I like those rugged human details in a 1960s TV series. Kirk really ribs McCoy, I kind of enjoy the interaction there.
What bugs me a LOT, and other people seem to forget, is that the alien is impossibly characterized. I have read that some people say that this is an episode that shows the inhumanity of the "Star Trek" universe - killing a being that is the "last of its kind". That would have some weight if the alien here was an "innocent" life form. But it doesn't come out in the wash. First of all, there is no hope for a creature that has no breeding partners. But most importantly, this is a being able to assume any identity, and that also can mimic any human's thought patterns to carry out its deceptions. Does this highly intelligent and capable life form just explain the situation and ask for salt to live on? No, it decides to stalk and kill the very people it needs to ultimately survive. Unlikely, incredibly unlikely. The only real conclusion is that the alien is insane, full of ability and thought - but stupidly homicidal.
I tend to believe that there was a good science fiction story underlying this episode, but that there was some pressure to make a "horrifying monster" - and so the story suffered as a result of that.