M-113 was an inhabited M-class planet. It was the homeworld of a type of poisonous plant known as the borgia plant, as well as the home of a civilization of M-113 creatures, which went extinct long before 2266.
The planet was hotter and more arid than Earth, so Human inhabitants needed to ingest larger amounts of salt to adjust for the loss of the mineral due to sweating. M-113 was once rich in salt but the planet's supply of the substance became depleted in antiquity, which was the cause of the eventual extinction of its native species of creatures, and by 2266, no salt could be found on the planet, so it had to be imported. Apart from the aforementioned borgia plant, M-113's landscape also featured dry brushes.
Several stone sculptures depicting various animals were found on the surface of the planet among vast ruins, including sculptures of a winged creature holding a sword, a small stone sculpture resembling a dolphin as depicted in Earth's Renaissance, a four-legged deer-like creature seated like a traditional Egyptian sphinx statue, and a sculpture of a lion-like animal.
In 2261, archaeologist Robert Crater and his wife Nancy began an expedition on M-113 to conduct a survey of the planet's many ruins. They moved into a large deserted ruin on the planet surface and began their archaeological research. During their first four years there, they shipped out large amounts of artifacts, gathered from several digging sites on the planet, made numerous reports, and were visited by a number of ships. A large drop in shipments was noted in their fifth year, however.
Unknown at the time of the Crater expedition, a single surviving M-113 creature continued to live on the planet. It eventually killed Nancy and continued to live along with Robert until 2266. Crater buried his wife on a hill on the planet, but was no longer as efficient in his archaeological work, causing the drop in shipments. (TOS: "The Man Trap")
By broadcast order, M-113 was the first planet ever seen in Star Trek.
M-113 has several connections to religious practices. In the final draft script of "The Man Trap", the planet was referred to as having once featured many "temples" used by the M-113 creatures, one of which was a large grey-colored building which, in the final version of the episode, is visible on the planet's surface. Since the name "M-113" (pronounced "M-one hundred thirteen") did not fit Star Trek's planet-naming conventions (and was originally suggested by Gene Roddenberry precisely because it was an unusual name), Kellam de Forest at one point suggested calling the planet "Mithras 113", referencing Mithraism.
According to Star Trek Maps (p. 31) and The Worlds of the Federation (p. 118), M-113 was the fourth of seven planets that orbited a yellow dwarf star, UFC 113. The M-113 creatures were in fact the ancient advanced civilization that had built the vast cities that were now in ruins. According to Worlds of the Federation, the indigenous name of the planet was Fotialla.
For the pre-remastered version of "The Man Trap", the brown/red planet model for M-113 was first used for Delta Vega in "Where No Man Has Gone Before". This planet model would be used for Alfa 177 ("The Enemy Within"), Tantalus V ("Dagger of the Mind"), Planet Q ("The Conscience of the King"), Benecia ("The Conscience of the King"), Gothos ("The Squire of Gothos"), Lazarus' planet ("The Alternative Factor"), Beta III ("The Return of the Archons"), Janus VI ("The Devil in the Dark"), Organia ("Errand of Mercy"), Mudd ("I, Mudd"), Argus X ("Obsession"), and Ardana ("The Cloud Minders"). For the remastered version of "The Man Trap", a new planet model was created by CGI artist Max Gabl.