Star Trek: The Man Trap

On stardate 1513.1, the Federation Starship USS Enterprise, commanded by Captain James T. Kirk, arrives at the planet M113 and sends down a landing party to check up on scientist Dr. Robert Crater and his wife Nancy (Dr. McCoy's old flame), who have been living alone on the planet for five years.
Without realizing it, each member of the landing party sees Nancy according to the power she exercises over their imaginations. For Crewman Darnell, Nancy resembles a voluptuous blonde he had met on Wrigley's Pleasure Planet. For McCoy, she presents herself as she was 10 years earlier, and to Captain Kirk she appears to be the same woman McCoy sees, albeit with more mature qualities.
The couple appears to be in good health, however, Dr. Crater wants to cut the visit short and appears to rush things along to get rid of the landing party as soon as possible. Before their departure, he also requests that they provide to him additional supplies of salt.
Soon thereafter, Nancy, in the guise of the voluptuous blonde, lures Crewman Darnell away from the others to a secluded place amid the rocks. Upon hearing a woman cry out, Kirk leads McCoy and Crater in the direction of the scream. They soon come upon Nancy standing beside Darnell's lifeless body, his face mottled with strange ringlike blotches. In the dead man's mouth Kirk discovers a piece of what Nancy calls a "Borgia" plant containing a substance similar to deadly nightshade, an alkaloid poison. Under questioning, Nancy claims to have warned Darnell when she saw him tasting the plant, but was too late.
After returning to the ship, McCoy initially can find no cause of death. He remarks upon the oddity of there being no trace of poisoning in Darnell's body, which, after further investigation, turns out to lack any trace of salt.
Kirk orders an additional security team to come down to the planet and begin an investigation. Shortly after their arrival, the creature kills off Crewmen Sturgeon and Green. Assuming Green's appearance, it joins Kirk and McCoy who soon discover Sturgeon's lifeless, salt-depleted corpse. Kirk, McCoy, and "Green" return to the Enterprise.
Once aboard, "Green" makes his rounds first entering Lt. Sulu's botanical laboratory after observing a salt shaker on the lunch tray brought to him by Yeoman Rand. "Green" hovers silently near the tray until startled by the shrieking of Sulu's pet plant. Catching sight of Lt. Uhura, the creature then transforms itself into a handsome black man and attempts to lure the lieutenant into its grasp, addressing her in Swahili, but a call from the bridge dispels the enchantment in which Uhura finds herself held.
Later, while scanning planet M113, Mr. Spock discovers no other life-forms save for Dr. Crater's. Surmising that Nancy must have somehow beamed aboard the Enterprise, Kirk orders a search for her. With the search underway, the creature manages to kill another crew member, then pays a visit to Dr. McCoy, appearing once again as Nancy. After the doctor falls asleep, the creature assumes McCoy's appearance.
Once again on the planet, Kirk and Spock discover Crewman Green's body and question Dr. Crater. Under pressure of their interrogation, Crater confesses that years before, the last member of a race of beings indigenous to the planet killed his wife. The being not only has illusion powers that allow it to appear in the minds of the viewer as something other than it really is, but it also an insatiable appetite for salt which it will kill to appease.
Kirk returns to the Enterprise along with Dr. Crater. Crater immediately recognizes the creature under McCoy's appearance, but does not alert Kirk. The two leave the room together, supposedly for McCoy to apply truth serum, but Mr. Spock accompanies them. The creature attacks them both, killing Dr. Crater, but Spock survives unharmed due to his unpalatable Vulcan physiology.
Awake, McCoy finds Nancy before him begging for his help, though to others she appears in her true form — a monstrous, misshapen and bulky creature with green flaky skin and a large, elliptical mouth. Still seeing her as Nancy, McCoy prevents Captain Kirk from trying to kill the creature. He then watches as it begins to suck the salt from Kirk's body. Spock returns and manages to free Kirk from the creature's clutches and tries to convince McCoy that he is not seeing Nancy. Spock punches the creature solidly, but it knocks him across the room turning once again to the helpless Kirk. The creature reverts to its true form as it begins to feast on Kirk, but freed from the illusion, McCoy kills it with a shot from a phaser.