Argelius II was the inhabited second planet in its star system. This system was located approximately 115 to 120 light years from the stellar cluster Jewel stars. The planet was the homeworld of the Argelians, a humanoid species. (TOS: "Wolf in the Fold"; TNG: "Datalore", "Conspiracy", production art)
Two hundred years before 2267, the Argelians experienced what they call the "Great Awakening". In this awakening, they united under an uniglobal government and created a legal system that was based on love. This government was headed by a Prefect. The seat of government was located in a chief city. Due to inefficiencies in their government, people from other planets were hired to fill certain administrative positions.
Prior to stardate 3619, Argelius II signed a treaty with the Federation wherein Starfleet could use the planet as a space port for their space vehicles and crews from the ships could take shore leave on the planet. The port had strategic value for it was the only one in its quadrant. For male crew members, the Argelian women were considered one of the highlights.
In 2267, the Prefect was Jaris. In that year, the planet was the scene of several murders committed by the entity known as Redjac. Captain Kirk and Lieutenant Commander Scott both enjoyed several visits to the planet, though Scott was framed for the murders during his first visit. (TOS: "Wolf in the Fold"; TNG: "Relics")
In 2364, the location of Argelius II was labeled on a star chart filed in the USS Enterprise-D library computer. The planets Mars and Alpha Eridani II were on a direct alignment between Argelius II and Earth. (TOS: "Wolf in the Fold"; TNG: "Datalore", "Conspiracy", production art)
"ar-GEE-lee-us" was the pronunciation for this planet's name from the script pronunciation guide for "Relics".
The planet model first used here for Argelius II would later be used for Sigma Iotia II in "A Piece of the Action", Troyius in "Elaan of Troyius", and Scalos in "Wink of an Eye". For the remastered episode, a new planet model was created by CGI artist Max Gabl.
According to the The Worlds of the Federation, the star that Argelius II orbits was named Argelius B (apparently part of a binary star system with Argelius A), and the Argelians referred to their world as Nelphia.
According to Star Trek: Star Charts ("United Federation of Planets I"; "United Federation of Planets III") and the Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), the Argelius (Iota Leonis) system was located in the Beta Quadrant. It was a binary system with two F-class stars. In 2378, Argelius II was listed as a Federation member.
The star chart seen in "Datalore" and "Conspiracy" originally appeared on page 133 of the Star Trek Spaceflight Chronology, where it was explained as showing a part of Federation space in the 2140s, with the thin lines denoting routes of commercial tourist starliners.
According to the novel The Sorrows of Empire, the mirror universe Montgomery Scott was likewise accused of brutally murdering three women on Argelius II. However, Captain James T. Kirk ensured that the charges were dropped without an investigation taking place.