An artificial S3 moon in Deep Space, the Psi-moon could "tune into the psyche" of any human or hologram astro that landed on it, rapidly re-configuring and landscaping itself accordingly to their mental state.
This could be a very dangerous place to be for the mentally unbalanced and deranged, such as Arnold Judas Rimmer, the Psi-moon's last visitor.
The Psi-moon was still operational three million years into the future, although dormant, and still awaiting a new mind to come to it. Eventually it was passed by the JMC mining spaceship Red Dwarf. The highly neurotic hologram Arnold Rimmer took the mechanoid Kryten down to the Psi-moon in a Starbug to claim it for the Space Corps, unaware of it's true nature.
Kryten planted a flag and they started to sing the 23-stanza-long Space Corps anthem. The moon then suddenly started to violently erupt and quake all around them, which for Kryten came as something of a relief from Rimmer's singing. However as the moon began rapidly terraforming, the quakes caused Starbug to shatter and collapse on top of Kryten, smashing and crushing him. The last thing he saw of Rimmer, apparently now given a solid body again whilst on the Psi-moon, being taken away on a cart by hooded figures with glowing red eyes.
Kryten sent his hand off in one of the 'bugs ore sample pods back to Red Dwarf, to fetch help from the others. Lister, the Cat and Holly came down in another Starbug, and Lister was forced to cut Kryten in half and put him back together to get him out. Kryten realises that it is a Psi-moon, which has become a dim, wooded, misty, dank and foreboding place filled with swamp and marsh fires; a reflection of Rimmer's twisted mind.
The Psi-moon had caused Rimmer's innermost neuroses, paranoias and feelings of failure to become solid as the small hooded figures with glowing red eyes, who kidnapped Rimmer and took him to the cave where dwelled the most powerful entity on Rimmer's Psi-moon: the The Unspeakable One. An imposing and grotesque creature, it was a manifestation of Rimmer's self-loathing and wanted nothing more than to tell Rimmer how much he hates him, before physically torturing him with an array of devious instruments, including a branding iron in the shape of the "H" on Rimmer's forehead. His Handmaidens proceeded to oil Rimmer, which Rimmer initially took for sexual foreplay, but was actually to increase his electrical conductivity.
Meanwhile, the rest of the gang journeyed through the Psi-moon's terrain and in a canoe through the swamps to reach Rimmer. The land was filled with strange animal noises and an unbearable stench. They travelled through the Swamp of Despair, the Wood of Humiliation and the Chasm of Hopelessness before encountering a graveyard filled with the corpses of dead aspects of Rimmer's psyche - Honour, Self-respect, Self-confidence, Generosity and Charm. Their was also a freshly dug grave reserved for Hope.
The Boys from the Dwarf managed to find Rimmer and rescued him from the Self-Loathing Beast, not with their Bazookoid fire (which merely irritated the Beast) but by making Rimmer feel better, when they assured him that they wouldn't abandon him even if it put them at risk. The Psi-Moon reacts to Rimmer's increased hopes, thereby diminishing the power of the Beast.
As they escaped and Rimmer was made aware of their dire situation, Starbug was trapped by quicksand. The Unspeakable One launched his legions (including Bitterness, Self-doubt, Mistrust and Loneliness) against Starbug, but the crew were able to raise Rimmer's sense of self-worth by telling him that they loved him and respected him. This caused his Self-confidence and Self-respect to be raised from the dead as the Rimmer Musketeers, who then protected Starbug from the hordes long enough for the ship to take off.
Whether or not the Psi-moon continued to be affected by Rimmer's mindset after they had left was never established, nor whether the crew left warning signs of any order for other spacefarers.