Stardate 1531.1: The Enterprise arrives at planet M-113 for an annual colony resupply and physical checkup of its residents and finds Professor Crater and his wife Nancy who is an old girlfriend of McCoy's. Crater immediately objects to the intrusion and tries to convince Kirk that an examination is not necessary and that their only need are salt tablets. Unknown to the landing party, each of them is seeing Nancy differently. McCoy sees her as the same youthful woman of his past while Kirk sees a hansome but older woman. Still, crewman Darnell sees Nancy as a blonde woman he met in the past and follows her into the nearby wilderness. Interrupted by the screams of Nancy, Kirk and McCoy race to the scene and find Darnell dead with Nancy standing nearby. Crater claims that the crewman died from the poison of the borgia plant found in his mouth, but Kirk is suspicious and orders everyone beamed up to the ship while he conducts an investigation. Unknown to the Enterprise crew, Nancy Crater is actually the sole survivor of M-113's native inhabitants, all of whom had the power to assume the shape of other life forms and to hypnotise their prey before killing them. Needing salt to survive, the M-113 creature killed Nancy for her body salt over a year prior to the Enterprise's visit, but the professor decided not to kill the creature because it was the last of its kind. Eventually, after several crewmen are killed on the planet and aboard the ship, Kirk and Spock learn of the creature and begin plotting its capture, but since it can assume the identity of other life forms, detection of the creature is difficult. Kirk asks Professor Crater for his help, but before any information can be obtained from Crater the salt creature, while disguised as McCoy, kills Crater and flees. Kirk corners the creature in McCoy's cabin but is overpowered by its hypnotic effect. Spock rushes in and strikes the creature until he himself is thrown against the wall. The creature returns to its true appearance and, horrified by the revelation, McCoy kills the creature before it can kill Kirk.