A salt-dependent alien lifeform that can assume any identity begins killing members of the crew in its need to appease its appetite for salt, which it completely drains from each of its victim's bodies. The alien initially poses as an old love from McCoy's past, who is a scientist at an archaeological dig on the planet the Enterprise is orbiting. Inevitably, the alien is unknowingly beamed aboard the ship.
Why NBC chose to air "The Man Trap" as the first episode of Trek instead of launching the series with its actual pilot, the better-paced and more textured "Where No Man Has Gone Before," is something of a mystery. "Man Trap" features a relatively nondescript plot that moves along slowly and features one particularly lackluster action sequence that begs for the bold, renowned Alexander Courage underscore but doesn't have it.
Much of the story follows the alien around the decks of the Enterprise as it takes the form of other people in preparation for luring more victims—scenes that don't benefit from nearly enough tension or suspense. Saving the episode is the respectable torment brought to Bones in the final showdown, which benefits from good portrayal of confusion on the part of DeForest Kelley; and good use of the cast as an ensemble overall.
Next episode: Charlie X