With Red Dwarf mostly abandoned and being eaten away by the Chameleonic Virus, Kryten and Kochanski devise a plan which involves entering a Mirror Universe where everything is opposite; negative becomes positive and a virus becomes an antidote.
Kryten builds a Prism Laser in the Captain's empty quarters on Floor 341 from technologies scavenged from Z Deck, and directs the laser at the Captain's mirror to create a dimensional gateway. Rimmer crosses over first, only for the prism to break and trap him in the Mirror Universe. Whilst crossed over, Rimmer finds that he is Captain of the alternate, mirror Red Dwarf, and has a little fun with the reversed roles before going off to find The Professor.
When Rimmer returns to his universe with the antidote printed off on paper, he finds the other Dwarfers have since repaired the prism and followed him into the Mirror Universe. By now Red Dwarf is a flaming inferno and falling apart. Rimmer tries to return to the Mirror Universe, but the Chameleonic Microbe has eaten away at the laser, turning it into a pile of grey goo. Rimmer tries to create the antidote, but The Chocolate Dispenser in the flaming corridor outside reveals that the formula from the mirror universe is now the formula for the virus, having reverted into its mirror opposite. (RD: Only the Good..., Series VIII)