The Man Trap was the first-aired regular season episode of Star Trek.
In this episode, a landing party from the Enterprise beams down to perform an annual checkup of scientist Bob Crater and his wife Nancy, who have been living on the planet M113 for 5 years. Without realizing it, each crew member sees a different Nancy Crater. In fact, Nancy's appearance corresponds to each man's fantasy of an extremely attractive woman. Crewman Darnell, for instance, sees a voluptuous blonde he met on Ripley's pleasure planet, Bones sees an attractive woman of 25, and Kirk sees an older woman with more mature charms.
Dr. Crater and Nancy both appear to be in good health, but Dr. Crater goes out of his way to request an additional supply of salt from the Enterprise's stores. Dr. Crater then does his best get rid of the landing party as quickly as possible. Shortly thereafter, crewman Darnell wanders off after Nancy Crater, but dies under mysterious circumstances. His corpse, when discovered, is found to be covered with a curious red mottling. A borja plant, belonging to the same family as the deadly nightshade and containing an alkaloid poison, is found in his mouth. Nancy claims to have seen him eating it and says she tried to stop him. However, Bones knows alkaloid poison when he sees it, and is immediately convinced that something is amiss. Further test show that Darnell's body is completely devoid of salt.
When an additional landing party beams down, crewmen Sturgeon and Green are killed and the salt is removed from their bodies. However, Green's body is not discovered, and Nancy Crater uses her shape-shifting trick to assume his form. As Green, she transports aboard the Enterprise with Kirk and Bones, who are the only remaining survivors of the landing party. ``Green'' makes his way into Sulu's quarters after observing a lunch tray with a salt shaker being brought in, but high-tails it out again when Sulu's pet plant begins shrieking at him. Green then transforms into a handsome Swahili man and attempts to seduce Uhura. Fortunately for Uhura, the man's advances are interrupted by passing crew members.
Meanwhile, a scan of the planet's surface reveals only a single life form, so Spock and Kirk realize that Nancy must have beamed aboard the Enterprise and start searching for her. While Spock and Kirk are thus engaged, the creature kills another crew member, and then pays a visit to McCoy in the form of Nancy Crater. She puts McCoy to sleep with his own sleeping pills while pretending to help him rest, and then assumes his form. In the meantime, Spock and Kirk have beamed down to the planet to question Dr. Crater. Here, they discover Green's body. By questioning Dr. Crater, they also learn that Nancy is dead and that her form has been taken over by the last remaining indigenous creature of the planet. As the audience has known for some time, this creature is capable of assuming any form and requires salt to live.
Kirk and Spock then beam Dr. Crater aboard the Enterprise, where he immediately recognizes McCoy as one of Nancy's impersonations, but does not reveal him. The creature returns Dr. Crater's loyalty by killing him, and then attacks Spock. Fortunately, because of Spock's non-human blood, he survives the attack without major injury. Bones then prevents Kirk from killing the creature (which he still sees as Nancy Crater), and then stands idly by as she begins to drain the salt from Kirk's body. At this juncture, Spock rushes in and demonstrates to McCoy that the woman attacking Kirk could not possibly be Nancy by striking her repeatedly and forcefully. Nancy does not even flinch, and then sends Spock flying across the room with a single counterblow. When the creature attacks Kirk again, its true alien form is revealed, and Bones kills it with a phaser, even after it reverts to Nancy's form.