The Enterprise stops at planet M-113 to resupply the scientist working there, Dr. Robert Crater, and to permit Dr. McCoy to perform a mandatory annual medical exam. Dr. Crater's wife, Nancy, is an acquaintance (and former romantic interest) of Dr. McCoy. When McCoy sees Nancy, she looks to him exactly as she looked when last he saw her, twelve years earlier. This is only the most innocuous mystery on M-113, though, for soon a crewman is found dead, his body drained of salt.

It turns out that a shape-shifting creature native to the planet killed the crewman, and years prior Nancy, out of a desperate need for salt. As it is the last of its species, Dr. Crater could not bring himself to kill it. In the end, though, McCoy is forced to kill the creature to save Kirk.

Notes

The substance of the story is in two main parts: first, the mystery of just what's happening, which is not too hidden from the audience; second, moral questions as to whether the lie that the Professor was living is worthwhile, and what is the right action to take with regards to the creature–Kirk compares it to the buffalo.

I'm reminded of the Coeurl from van Vogt's Black Destroyer.