Tarchannen III was the third planet in the Tarchannen system. The planet was home to the Tarchannen III species.
The planet was characterized by perpetual twilight, frequent sandstorms and its sparse vegetation, including a variety of dark, spindly plants. The planet's atmosphere stretches up to an altitude of 210 kilometers.
A Starfleet outpost consisting of five buildings was located on the planet, until the whole crew of 49 disappeared in 2362. Their disappearance was investigated by an away team from the USS Victory, led by Lieutenant Susanna Leijten. The outpost showed no signs of an attack or a natural disaster – the people had just vanished.
Graphic of a plant native to the planet
It was not until 2367 that this mystery could be solved. Three members of the former away team suddenly deserted and attempted to return to Tarchannen; one of them, Paul Hickman, was killed in this attempt. While the USS Enterprise-D was investigating these incidents with Leijten on board as an observer, both she and the last away team member, Enterprise chief engineer Geordi La Forge, underwent a strange transformation. Leijten was kept in sickbay, while La Forge managed to transport down to the planet, close to Building One. It was discovered that their transformation was caused by a parasite which planted its DNA in living hosts to reproduce itself; the hosts then transform into a primitive reptile-like species with the overwhelming urge to return to Tarchannen III. La Forge and Leijten could be rescued and retransformed, but there was no hope for the others.
Following this discovery, warning beacons were placed in orbit and on the surface of the planet to prevent similar incidents in the future. (TNG: "Identity Crisis")