The Dimension of Rain is an alternate, darker version of Sluggy Prime. It is almost constantly raining (at least where Riff is) and apparently covered in an endless desert. There is only one known break in this desert and that is the Dystopia known as 4U City. The rain of the dimension usually only lets up once every thirty days, but in recent strips the dry spells have been coming more frequently and lasting longer. (it is unknown why or what the significance is)
4U City's history is at once similar and different than that of Sluggy Prime. Torg has apparently never met Oasis in this dimension, and Gwynn has never been mentioned. Torg still loves Zoë, and she was lost in some sort of accident, and that dimension's Riff was supposed to bring her back to him. Also Riff of that dimension was married to a woman known as Isabella. After he left, a cache of his inventions was found by Hereti-Corp. Shortly after a war broke out between the various supervillain corporations known as the R&D wars. At least one alliance between Crushestro and NoFun produced dangerous armed mutants which subjugated Asia in a matter of months. After the war Hereti-Corp, headed publicly by Dr. Schlock, combined with several groups and formed 4U-Corp. They built 4U City and fluxed out the rest of the world's inhabitants using Riff's inventions. According to alt-Rammer's love interest Sharon, this event broke the web of fate in this dimension. Since then His Masterness (believed by this dimension's Torg to be Schlock) has ruled 4U City with an iron fist. Some of his laws include a standing kill-on-sight order for anyone with Riff's DNA, and an order to take anyone with Zoë's DNA into custody. After Riff from Sluggy Prime arrived, alt-Rammer saved him from being killed and recruited him to start an uprising against 4U City. He did this for revenge for Sharon's being fluxed for having had contact with Riff. Riff is currently leading the resistance to destroy 4U City and is trying to find a way to get to Zoë and "set her free" from the pain she is in.
When 4U City was created its leader, His Masterness, used a weaponized version of Riff's DFA to supposedly flux every other being in this dimension into others, in order to end and win the R&D Wars. Some of the armies from the R&D Wars have been fluxed into this dimension from parallel dimensions, they are known as outsiders, live outside the city and regularly attack. Other than them the only inhabitants of the dimension reside within the confines of 4U City.
Everyone in 4U city (excluding the executives and His Masterness) are called by their citizen number. This sometimes makes things confusing, so here is a handy list of citizen numbers, and their identity. Most of the citizen numbers spell out a word on a US touch tone phone.
A list of all persons in the Dimension of Rain can be found on the Characters site.