Now, where to start?
The original concept of the show was intended to be about a kindhearted optimistic (and for all we know probably Mormon) young boy (a.k.a. Jimmy) who one day got hit by bus. (supposedly he save an old granny in the process) Luckily for him, the animation he's in undoubtedly has a heaven and hell. Better still, he was good boy and earned a spot in heaven. Unfortunately, God/Jesus/whatever powers that may be up there in his animation stated that his death was a complete freak accident (and they just happened to have a shortage of chairs) and was sent to purgatory to wait out his time that he was supposed to still have on Earth. Next thing you know our overly upbeat main character is having to make hell his home and the Devil (Lucius) isn't ecstatic about it and tries his damnedest to break the goofball's spirit before he goes back to where he belongs but with little to no progress, and one day he even goes as far as summoning the soul of a sadistic knife happy serial killer (Heloise) to pitch in on causing harm to the poor boy. (who by the end of said episode was going to turn on Lucius and start developing a one sided love/hate relationship with Jimmy)
This however was as stated "the original concept" and was drastically altered for obvious reasons. (even though it would have arguably been far more awesome)
The concept of the show had kept some things the same. Such as Lucius being a tyrannical ruler and Heloise being his right arm but out right changes others and muffles a few ideas. (which isn't exactly accept as fact but not disproved either) Let's see:
The residential area Jimmy now lives in is no longer Hell/Purgatory but "Miseryville" which is located on an alien planet. It's residents are also mostly anthropomorphic monsters (who can't be labeled as evil) instead of sinners.
There is absolutely no explanation of how Jimmy wound up in Miseryville but it is clued in that he's undeniably foreign to the place.
Heloise as far we know was never a knife happy serial killer and wasn't summoned by Lucius to torture Jimmy and had been friends with the said character since the first episode. She's still a sadistic little girl but trades in her knife (and supposedly her originally ghoulish appearance) for the an unrivaled I.Q. with ingenuity matching Dexter from Cartoon Network.
As for Beezy? To my knowledge it was never stated how he met Jimmy in either the original or final concept. So you got me on that one.
tl;dr: It's Spongebob turned human and anonomysly thrown into psuedo hell.
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