A shape-shifting creature on planet M-113 killed archaeologist Nancy Crater, took her form and drained the salt from Enterprise crew members until McCoy killed it.
In 2266 the Enterprise made a routine medical call at planet M-113, where archaeologist Professor Robert Crater and his wife Nancy were studying ruins. Nancy had once been romantically involved with Leonard McCoy. The landing party did not know that the real Nancy was dead and that the woman they met was the last surviving native of the planet, a creature that could take any form and fed on salt.
Crew members began dying with strange marks on their faces, their bodies emptied of salt. Crater had protected the creature out of loneliness and lied to the landing party about it. Disguised as McCoy, it came aboard the Enterprise and attacked Kirk. Spock exposed the deception and the real McCoy, seeing it in its true form, shot it with his phaser.
The crisis raised hard questions about killing the last member of a species. Kirk compared the creature to the buffalo of Earth, but the danger it posed to the crew left no other choice. The episode depicting these events was the first broadcast of Star Trek: The Original Series in 1966.