Nancy Crater
Jeanne Bal

Professor Robert Crater
Alfred Ryder

Darnell Green
Bruce Watson

The 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.