A creature that feeds on salt impersonates a former love of Dr. McCoy and stalks the crew of the Enterprise.
The Enterprise visits planet M-113 for a routine medical check on archaeologist Professor Robert Crater and his wife Nancy, a woman Dr. McCoy once loved. Soon crew members begin dying with the salt drained from their bodies.
The real Nancy is long dead. Her place has been taken by a shape-shifting creature, the last of its kind, which can appear as whoever a person most wants to see, and Crater has been protecting it. The creature gets aboard the ship, and McCoy is finally forced to kill it while it wears Nancy's face. Broadcast on 8 September 1966, it was the first Star Trek episode to air.