A desolate planet holding the ruins of a dead civilization, home to the Craters' archaeological dig and the last of its salt-craving natives.
M-113 is an arid, dusty world whose ancient civilization disappeared long ago, leaving ruins that attract the archaeologists Robert and Nancy Crater. Its native inhabitants, shapeshifting beings that feed on salt, have dwindled away, apparently as the planet's natural salt deposits ran out.
In The Man Trap the Enterprise visits M-113 to deliver supplies and give the Craters medical examinations. Crew members begin dying with their bodies drained of salt, and Kirk learns that the last surviving native has killed Nancy Crater and taken her form. The planet's role ends when the creature is destroyed aboard the Enterprise.