The U.S.S. Enterprise reaches the planet of M-113 with supplies for Dr. Robert Crater and his wife, Nancy. Doctor Leonard McCoy had once had a romantic relationship with Nancy. The Craters have lived on M-113 for five years, during which time they've done an archeological survey of the planet's ruins. They are the only known inhabitants.

Crater tells Kirk that they only need salt tablets, and he wants to be left alone. Kirk disagrees, insists they must require more than that, and wants McCoy to examine them. During all this, one of Kirk's landing party, Darnell, encounters a gorgeous woman and walks away with her.

Kirk searches for the young man but sadly discovers him dead, bearing suction-cup-shaped wounds. He orders the Craters to beam up to the U.S.S. Enterprise as a safeguard. Upon arrival, Dr. McCoy is astounded at how little Nancy has altered since their last encounter several years ago. Her close presence on the ship brings back McCoy's former emotions, which obstruct any doubts he might have held about her.

Nevertheless, the woman McCoy recognizes as Nancy Crater is a shape-shifting entity, the last one left of M-113. This being can take the physical form of a different person to each individual it encounters. By delving into their minds and using their recollections, the creature can construct a sense of security in its victims before ending their lives.

The M-113 creature is in peril due to the scarcity of sodium chloride on its home planet. With the salt unavailable, its species is doomed to extinction. Fortunately, the crew of the Enterprise has a source of supply: themselves. Unfortunately, all of its kin died due to this deficiency, and the last remaining M-113 formed a symbiotic relationship with Professor Crater. In exchange for providing the creature with salt, it gave the professor companionship, something Crater desired following the murder of his wife, the actual Nancy Crater, for the purpose of obtaining her salt.

The M-113 creature, which was loose on the U.S.S. Enterprise, started by killing crew members by posing as someone known and trusted to them before it drained them of their body salt. Ultimately, it murdered Crater and then took the form of Nancy Crater, nearly killing McCoy. Kirk and Spock, who had figured out the creature's true nature, quickly went to the doctor's quarters and convinced him that this was not the honest Nancy. In an emotionally wrenching move, McCoy ended up slaying the creature and saving himself and the U.S.S. Enterprise crew.