AllMovie

The Starship Enterprise makes a routine call at the arid, uninhabited planet M-113 in order for Dr. McCoy (DeForest Kelley) to give a required annual medical check-up of the husband-and-wife team of archeologists working there. One seemingly minor complication is that Nancy Crater (Jeanne Bal), the wife, was once involved romantically with McCoy. Captain Kirk and Dr. McCoy meet with an irrationally hostile reception from Robert Crater (Alfred Ryder), however, and then members of the crew start turning up dead from unknown causes. Ultimately, it's determined that they were killed by a sudden and medically inexplicable loss of all the salt from their bodies. The mystery deepens as some crew members find themselves approached by mysteriously compelling strangers, and the planet below is short one of its two inhabitants. Eventually, Kirk and Spock determine that their adversary is the last native inhabitant of the planet, a kind of salt "vampire" that has the ability to alter its appearance to the shape of whoever its victim most wants to see. Needless to say, finding the vampire aboard the ship is easier said than done. Its shape-shifting ability has also allowed it to temporarily incapacitate McCoy, whose ignorance of events nearly costs Kirk his life.