I don't know it too well, but it runs something like this:
The tosser who later called himself the Emperor was born of the gestalt consciousness of a load of tribal witchdoctors in Asia Minor in either 4,000 or 8,000 BC (I forget), in order to defy the Chaos Gods (frickin' alien consciousness weirdo freaks. He hung around for a while for no discernible reason, watching humanity like some weird perv. Some tens of thousands of years later, humanity had colonised much of the galaxy, then the Chaos gods pulled a hissy fit and shut down interstellar travel - which works by magic, in essence - for a while.
Then the Emperor decided he'd had enough of doing nothing, created a score of superhumans with more personality flaws than a barrelful of paraplegic monkeys that have been abused by chimps for a year, planning to use them as generals in a campaign to conquer the galaxy. The Chaos gods pulled their thumbs out of their collective arse, and teleported these superhuman idiots all across the galaxy to a load of stereotyped locations. One, for example, ended up on Ancient Rome World where he had to fight as a gladiator, until he pulled a Spartacus. More of him later.
Having lost his shiny toys, the Emperor got narked, conquered Earth anyway, banning religion in the process because belief in any god
might make the Chaos twerps stronger - don't worry, this idiocy comes round to bite him in the arse later. The Chaos gods have by now given up on screwing about with interstellar travel, so the Emperor ships out a load of superhuman blokes in power armour to conquer the galaxy. Priority #1, however, is to recover the Primarchs - the score of superhumans who'd keep Freud in Oedipus complexes from now until Doomsday. So having conquered a load of places with no trouble whatsoever, he recovers the Primarchs, installing them as generals . . . regardless of what they were doing beforehand. For example, I'm-not-Spartacus above was about to have his arse handed to him by the locals, when the Emperor teleported him out and refused to let him command or help his army, claiming vacuously that there was no time.
So having turned over command of his massed legions to a load of lunatics who shared his genes but had either way OTT feelings of love for him or hated him for being a twat, he pottered off back to Earth, letting his favoured son, Horus (ooh, is he going to betray his father's trust lamely?) take care of conquering the rest of the galaxy. The Emperor, being a genius, and fully aware that the four Chaos gods, Slaanesh (perv-cum-Varoto), Khorne (me angry!), Nurgle (doesn't wash his hands before meals) and Tzeentch (you'll like this! Not a lot, but you'll like it!) are none too keen on his schemes, then proceeds to ignore evidence of some of his supermen in power armour being murderous nutters and so forth. So a load rebel, planning to knock him on the noggin. If he doesn't ignore the evidence of inevitable rebellion before they rebel, he paranoidly imagines it in some after others do rebel.
Fully half his superhuman nutbars rebel, screwing up his plans to unite the galaxy against the mean old Chaos weirdos. Loyalists fight desperate rearguard actions while the rebels charge headlong toward Earth, landing and attack the Emperor's fortified palace with superweapons - the size of the guns of which would make Hitler feel positively embarrassed about considering anything as small as a gun capable of firing from mid-France to London.
So the Emperor - genius psychic dude, remember? - sees half his generals go mad and try to kill him, smash up his pretty star empire like a load of genetically enhanced bulls in a china warehouse, watches them shaft Earth's defences and land to poke his Loyalists (idiots) in the eye with a blunt stick fired at 7,000 mph from a gun the size of Paris, and thinks to himself, "Neah, I'll just have a chat with Horus. We can straighten this all out." Genius, this Emperor. I'd never have thought of that plan, you see, so it must be right clever.
So Horus turns up, pokes the Emperor a bit, and then the Emperor, while almost dead, says to himself, "Duh! I've been a right tosser!" Zaps Horus the Dumb with Force Lightning, shouts boo at the psychic shades of the Freaky Four, and lapses into a coma. The Rebels, being lame as heck, fall into confusion at this point, inexplicably evacuate Earth and double-time it to the centre of the galaxy, where dwell the Chaos gods. A better plan might have been to kill the rest of the Emperor's guards, and chop up the body into little bits, but I'm no genetically enhanced super-warrior with years of campaigning under his belt.
Anyway, they bugger off. The Loyalists are all "OMG! He's like, dead, dude!" "No way, man! He's still breathing! Quick, let's stick him in this futuristic iron lung doodah!" "You mean life support machine?" "What-evah!" So they bung the body in a box which amplifies the Emperor's thoughts, and set about repairing the ruins.
Following the Emperor's death a couple of things of note happen. First, everyone becomes utterly brain-dead. New technological innovations are a social faux-pas on the scale of wearing leopardskin-print codpieces to a formal dinner. Second, massive irony kicks in, and the Emperor who forbade the worship of any gods is worshipped as a god. Humans all over the galaxy do this. The superhumans he created, the Space Marines, know that this is not what he wanted, but they don't complain because, er, because of the magical element of handwavium.
Ten thousand years pass, and now the galaxy is a slightly different place. Humans are all mediaeval morons with lasers who occasionally fly through space in several-mile-long Gothic cathedrals outfitted with big-ass lasers and torpedoes the size of 747s. The galaxy's been invaded by a load of extragalactic dudes who make the Aliens in
Alien look like knitting champions.
The Rebel Space Marines in the middle of the galaxy occasionally pop out for jaunts. When they d have an excursion they make a point of hitting planetary populations with big chainsaws tied to swords or axes
or inflicting 'orrible diseases that make everyone feel and look as if they had sex with the entire sex-worker population of the planet in an hour then raped a heretofore undiscovered species of ape in the African jungles, thus contracting a new disease that combines all the least photogenic aspects of Ebola, AIDS, the Black Death and acne
or having the most painful, implausible and painful sex possible in a way that makes that bit in
Seven where the bloke stabs the poor old prostitute with a stabby-stabby strap-on dildo seem positively relaxing
or they pull lots of rabbits out of hats, and ears, and noses, and melt one's eyeballs, and then turn them into pizza, then nibble on 'em, then leave 'em in the fridge, then reheat 'em in a microwave the next day, then turn 'em back into eyeballs, so you can watch as they melt you into a pile of boiling books. Anyway, Ted Avery's wildest cartoon visions have nothing on these whacked-out loons.
The Loyalist humans, being The Good Guys, do all of the above except the last bit, 'cause that's magic, and thus Naughty. They also send armed cyborg librarians round the galaxy looking for blueprints for tanks and stuff that they dropped down the back of the sofa some millennia ago. Careless.
Space elves are also pottering about. They have a military like Switzerland, really don't like the pervy Chaos god, have a Hyacinth Bucket attitude toward the humans, and are, in short, stuck-up idiots. They fight anyone they feel like, justifying their actions on grounds of spooky prophecies, and storing the souls of their dead in rocks in their space-cities.
Even in the future, sadly, the International Communist Conspiracy has not died out. Indeed, the Tau, so named because they have no discernible collection with the Greek alphabet, are probably the most likeable species out there, since they're the only race that doesn't blow up planets when they're feeling bored.
There's also the Undead, repackaged as the
Enemy From Before The Dawn Of Time! The Necrons, metal skeleton dudes who live in pyramids, have a sinister and nefarious plan to do something or other probably involving blasting a hole in the side of Mars so as to free an ancient dragon-god-thing. Don't ask.
Anyway, they've been around forever, and had a big bust-up with the Space Elves quite a while ago, and so they don't get on. Their lone plus-point is that they're the only ones who freak out the Extragalactic Alien Menace.
Last and either least or first, depending on your feelings about football hooligans, are the Orks and related races. Some super-intelligent race designed the Orks to colonise everywhere and serve them. Said race was so very intelligent that it promptly got itself wiped out by some disease. The Orks' response can be summed up as "Bugger. OK, who fancies a beer at the next star system along? Last one to get a black eye in a punch-up's a pooftah!" They fight everyone and everything, mainly themselves, but they'll also take on anyone else nearby. They're big green asexual humanoid sentient mushrooms in appearance, and probably taste delicious pan fried in olive oil with some garlic and onions.
Hitch-hiker's guide to the 40k Galaxy