The Underplex is a compact Paranoia supplement which details the “spaces in between” – the abandoned corridors and levels that lie between, under and over all the inhabited areas of Alpha Complex. As such it’s a fun and natural idea, and quite useful as a place where secret society meetings could sometimes take place… and also as a shortcut between point A and B, where nasty and lethal thing C is blocking the usual route. Naturally enough, it also serves as an excuse to throw new hazards in the PCs’ direction. Where do all the hungry & dangerous mutants live? Why, in the Underplex of course!
The book contains ideas on how to populate these “uninhabited” areas and also presents an “adventure seed” on the same subject. This is by no means a “must have” book for Paranoia, but neither is it bad. Sure, you can and should just invent most of this stuff yourself… but getting extra ideas is rarely a bad thing.