Star Trek: The Original Series, season 1, episode 1
The Man Trap
Summary
The Enterprise visits planet M-113 for routine medical examinations of the husband-and-wife archaeological team stationed there, but the woman has been replaced by a shape-shifting creature, who survives by assuming a new form depending who it meets and extracting the salt from the bodies, killing them.
Summary from Wikipedia, CC BY-SA 4.0.
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