The Rejuvenation Shower is a technology aboard Red Dwarf discovered by Kryten and used by the Boys from the Dwarf. In theory, it was used to restore youth in a human, although if installed incorrectly it could be used for time travel. However the latter only occurred erratically, and this form of time travel was unstable.
Kryten collects the Dwarfers in the sleeping quarters and presents his find: a rejuvenation shower, which is capable of rewinding in time any living being's genome back to its physical prime. The crew are intrigued, until Rimmer points out that it is a Swedish made flat-pack self assembly and they have to build it themselves. In a bungle of parts and effort, the crew get to work. Lister eventually proclaims it to be done after complaining that they had taken it apart and rebuilt it "over a hundred times", despite the Cat's insistence that there are too many spare parts still lying around. Deciding that they should test it on something unimportant first, they place one of Lister's more infested socks into the shower. Once the machine is activated however the beam extends farther outward than it should due to a lack of alignment brackets. The crew find themselves encased in "beamer light" and are teleported away from Red Dwarf to a distant forest. Kryten checks his internal chronometer and finds that they have reached the land of Albion (or Britain) back on Earth in the year 23 AD.
The instructions read: "With just an Allen key and a Phillips-head screwdriver, assembly should take no more than three hours. Please consult diagrams 1 through 369 for further instructions. Do not forget to install the alignment bracket." ("Lemons")