Professor Robert Crater
Enterprise makes a call on planet M113 in order to carry out routine medical
examinations on archaeologists Nancy and Robert Crater. McCoy and Nancy
have known each other in the past and Bones is amazed to see that she
looks the same as she did when they last met ten years previously.
McCoy's comments are treated with a degree of doubt by the other crewmembers
as each one of them sees her in a different way. This is due to the fact
that the real Nancy Crater was killed by the last surviving creature of
a native species that has the ability to change its appearance in the
mind of observers. Kirk and McCoy are a little bemused by the couple's
understandable resistance to taking the medical examinations and their
insistence that all they need is a new, larger supply of salt tablets.
A crewman dies while walking in the ruins on the planet, apparently after
eating a poisonous fruit, but McCoy's autopsy reveals a complete lack
of salt in the man's body. Fearing for the safety of the Craters, Kirk
insists on beaming them up to the Enterprise while they investigate the
incident further, but Robert Crater hides from them and Nancy disappears
into the ruins.
A landing party is sent down to search for them and the creature murders
another crewman, this time taking his 'form' and returning to the ship
with the rest of the party, hoping to find new victims. The crewman's
body is discovered and the Enterprise is put on a security alert. Salt
is laid out as bait in a trap for the creature which, recently fortified,
is able to resist. It assumes the 'shape' of McCoy but makes a fatal error
when it tries to attack Mr. Spock who is immune as his Vulcan body salts
are based on copper.
The creature resumes its 'Nancy' appearance when Kirk and McCoy arrive
on the scene to kill it, and it pleads for help from the doctor. Unsure
of things, McCoy tries to protect it until, weakened, it reveals its true
form, at which point Bones has to accept the truth of what has happened
and forces himself to shoot what appears to be his old love.