The Man Trap (Episode)
Stardate 1531.1: A shape-shifting, salt-craving creature terrorizes the crew of the Enterprise.
The Enterprise visits planet M-113 to give archaeologists Robert and Nancy Crater supplies and medical checkups. Years ago, Nancy was romantically involved with Dr. Leonard McCoy—whom she nicknamed "Plum."
Now she is literally a different woman: each member of the landing party sees Nancy differently because the real Nancy is dead, killed by a shape-changing alien which has assumed her identity. The creature, the last surviving native of this planet, has been enjoying a symbiotic relationship with Professor Crater; Crater supplies the salt that sustains the entity, and the creature provides Crater with companionship.
The arrival of the Enterprise landing party upsets the delicate balance of this strange alliance. Craving additional salt, the creature kills two crewmen and boards the Enterprise disguised as one of its victims. The creature assumes Dr. McCoy's appearance before Captain Kirk eventually tracks it down and kills it with the aid of Mr. Spock and the real McCoy.