We're playing as students at a wizarding school somewhere in the Planescape setting. Here’s what one eyed Bob the street vendor has to say about the Planes:
What are the planes? Mind-boggling is what they are. You've got so many planes of existence all competing for dominance that you can't ever know what to expect and no one can ever really know what's going on. Your world, that little planet you're from, that's just one little place among a lot of really big places. But they're all connected, right? I mean, you got here from there, so they must be, and from here you can get anywhere, so that's all connected too, see?
Where are we? We're in Sigil. Yeah, I know it's a little strange at first, but don't worry, live here a hundred years and, well, it'll still be strange. Just between us I don't think it's natural, but that's not really important, is it? It's here, and it's been here near to forever, and it's the greatest city anywhere.
Oh, well, yes, Sigil is somewhere. Sigil's smack in the middle of the Outer Planes. You know those alignments the priests yammer on about? "Oh, your god's lawful good and so is mine, so there's no reason we shouldn't get along just fine," just before they brain each other with their holy mauls. Well, out here that sort of crap actually means something. We're living floating in the sky above an infinite expanse of intellectual territory. You've got one plane in the outer planes for each alignment, all surrounding the vast plain of true neutral. And no, those nature worshiping nuts are full of crap. Nothing's in balance, at least not for long. The devils are in an eternal war with the fiends, and the gods all hate each other. They're like children with really big sticks. They do tend to stick together in their little clans, but even so they scheme endlessly. The Lady, she's the one that rules this place, if you can really call it that when she hardly ever does anything, keeps all the gods and their divine magic out of here, but their servants come and go, so their fights find their ways in here too. So long as they don't threaten the existence of the city or the lives of too many of the citizens, she lets it slide. Kind of an anarchy with big scary cops lurking in the shadows. But everywhere else is worse. Everywhere else is someone's turf, and they might demand your service or worship, and their enemies might invade any time and kill you for no good reason.
What else do you need to know about the Outer Planes? Let's see... This isn't the only city out here, just the biggest. Any race you've ever met, they're here, and more besides. Like I said, your planet is just one little place, with a lot of other little places just like it. There are countless tiny planets out there, and they've all got people, and someone's here from every one of them. Most people are just people, but there are a few you need to steer clear of. The Gith are dangerous. Some of them will kill you for one religious reason, and the others for another, but dead is dead, right? They'll both kill you if you get in their way or piss them off. Take my advice and don't try to talk religion with them unless they bring it up, and never get between the two groups. Trying to figure them out might get you dead. And the Illithid. They drink minds and eat brains. Seriously. Every single one of them is a murderer, and they still wander the streets free here. Biggest slave dealers anywhere, and a pretty big trade in psychoactive substances too. In public you're safe, and in Sigil you're probably safe, but anywhere else, run. They threaten you, go find the Gith, both groups will gladly kill any mind flayer they can get their hands on.
Sure there are other planes. These are the outer planes, so there've got to be at least middle and inner planes, right? Well, yours is kind of the middle. The Prime Material Plane. All those little balls of dirt. They're linked to here by the Astral Plane. That's mostly a place to either use to travel from one place to another or to stick something. Then, kind of kick it sideways and you've got the Ethereal Plane. That's the very stuff of magic. It's kind of everywhere, but at the same time it's its own place. I don't really get it. Go find an Archmage and he can explain it better. Just like out here we're living on the embodiments of ideas, there are other conceptual things there. Inside the Ethereal Plane there are the Elemental Planes, the Inner Planes. You've got Earth, Fire, Positive Energy, Negative Energy, blah, blah. Elementals are from there, and so's the whole fabric that your planet's cut from. There are other things than the normal elements too. Ever met a Dread Elemental, made of dread like an earth elemental is made of earth? No, well, they're out there and they're nasty business. Elemental plane of Dread. I've never been there, and they say you can't ever escape it, but if that were true, how would we ever hear about it?
Listen, you want to get a good education in the Planes and learn some top rate magic, you go enroll in Hedgemere Academy. You go down that way, turn left at the second street, right at the third, and about a quarter mile down on the right you'll see a rusty iron door. Go through that door and you'll be in a back alley in the only city on a planet like yours, or at least a planet, so how different can it be? All those Prime places are the same to me. Ask around there and you'll find it. Good school. I took a semester there. Didn't have what it takes to cut it as a magician, but I learned plenty. No, don't worry, they'll get their payment out of you one way or another. They never worry about it. In the Planes there's always money for a bloke crazy enough to try and get it, and they use the student body like slaves that way. Nope, not too dangerous, usually, and part of the education. You learn a lot when a godling's trying to kill you for taking his shiny baubles. The lectures are pretty good too.
Go on. It'll be fun. Trust me.