The Gemini 12 was a mysterious spaceship capable of time travel and originating from the 28th Century. According to the Gemini 12's black box, her ill-fated maiden voyage was a covert reconnaissance mission to the 20th Century, during which the unfortunate crew inadvertently contracted a 20th century influenza virus. Apparently all viruses had long since been wiped out by their time, and so by the 28th century the human immune system had become so weak through inaction it no longer had the ability to combat illness. The crew of the Gemini 12 knew they were dying and so, to stop the time machine aboard falling into the wrong hands (knowing how it could be abused and how dangerous it could be to mess with time) they sent the ship away into deep space on autopilot.After falling into orbit around a gas giant, the Gemini 12 activated its unusual security system — a giant enveloping gas cloud which contained a "reality minefield". This consisted of temporary bubbles of "unreality" which would confuse and disorientate any trespassers and deter them from delving deep into the gas cloud surrounding the Gemini 12. The effects of these "unreality bubbles" included making Dave Lister, a human crew member, appear to be a robot; making the Cat become invisible and removing his existence from the memory of the other crewmembers; making Starbug appear to disappear around the crew; and the heads of the crew transforming into the heads of animals. As any ship penetrated deeper into the gas cloud, the effects of the "unreality bubbles" would become more and more unnerving, more ridiculous and more extreme.
Starbug went through the unreality bubbles, with the crew in cryogenic sleep so they wouldn't be affected by the unreality bubbles, to find out what they were protecting. When they found the Gemini 12 they took the Time Drive and installed it on Starbug. Soon after they came into a conflict with their future selves from fifteen years from then (who had grown incredibly decadent and amoral through abusing the Time Drive) which concluded with their future selves killing them. This caused time to reset to before they discovered the Gemini 12. However the time paradox of the battle with their future selves caused severe dimensional anomalies, which in turn caused the appearance, size and properties of Starbug, Gemini 12 and the Time Drive to alter greatly. Also the Dwarfers retained all memory of the meeting and subsequent battle with their future selves, which should have been enough deterrent to leave the Gemini 12 alone. It was not so...
They once again went and retrieved the Time Drive afterwards on Lister's insistence, so they could go back in time to Earth and replenish their curry supplies. To get the rest of the crew in on it, Lister swapped Kryten's normal head with spare head 2, which was less morally upstanding than Kryten's normal head.
The crew accidentally warp to Dallas in 1963, and get caught up in the assassination of President Kennedy. The Dwarfers manage to set the timeline straight but not after nearly bringing the Earth to nuclear holocaust. Afterwards the crew finally decide to no longer use the Time Drive as it proves too much hassle, or perhaps it was faulty or broken, which is implied during the episode "Tikka to Ride". As such the Gemini 12 is not seen again. Although not mentioned, the red flash of light and sound effect of the Time Drive is used in the background of the next episode ("Ouroboros") when Lister places the child of himself and Kochanski under the pool table where Lister was originally "found" as an orphan.
The Gemini 12 has had four different appearances, none of which are compatible. For the first episode it appears in, "Out of Time", they weren't able to build a new model and instead used stock footage from the episode "Justice". For the next episode, "Tikka to Ride", there was a failed attempt to build an all-new CGI design before it was decided to use another ship that would appear later in the same series, the SS Centauri. Finally, for "Tikka to Ride Remastered" on the Red Dwarf Series VII DVD, an entirely new design was made using CGI.
