Okay. I might as well give this a shot. I've not slept yet so expect a lot of typos/bad grammar, if a sentence MAKES NO SENSE lemme know. Here's the Gospel Acccording To Sarevok (you can basically take this as the actual story because I'm just paraphrasing (extending???) what's already in the in game + secret logs):
It all kick starts with The Black Marker arriving/being found somewhere, I can't remember where it's not really important. The Black Marker is a completely alien artifact and nobody really knows what it does or is for. The guy who 'started' the Church of Unitology was doing some secret research, which was halted and 'taken over' by the government. Again, I can't remember what this was to do with. It's not important. (I may also be adding in the plot of Imperium Galactica 2 as I've just played that and it deals with a VERY VERY similar plot...).The point is that NOBODY AT ALL knows what The Black Marker does, not the Unitology guy (who is assassinated when he leaks information over it, the resulting backlash founding the Church proper), nor the government, nor any scientists.
The Church of Unitology has the belief that death is the path to redemption, basically they're the Dusties from Planescape: Torment, if you played that. They believe when you die, your body will undergo a change and you will 'unify with the one true whatever', basically the Annelids from System Shock 2. Ironically their belief (which is completely unfounded and based on a random book of no import whatsoever) is the EXACT thing which happens when people get necromorphed. It would make sense if it was AFTER discovering the DNA in the marker (see below), but no they get their beliefs before hand. Way to have a shitty backstory. They are also taking all their dead members bodies and hoarding them on spaceships (and although it's never mentioned, most likely experimenting on them with the DNA from The Marker).
Now in the 'secret logs' which you unlock when you beat the game (and can only read on a second playthrough with the same save) there's mention of how it all begins, which combined with after logs throughout the game makes senes (Nicole doesn't make sense). They tried to reverse engineer The Black Marker in order to to see if they could understand what the fuck it was, and if it was useful at all. They knew it 'gave off' certain brainwave affecting patterns, but didn't know what they did. In the process of doing so, with the newly created Red Marker (which is identical in every way except for colour (aesthetics purely)) they discover a microorganism DNA. To jump ahead a bit, the Marker is basically a device to prevent the spreading of necromorphs, however due to the nature of this a trace amount of dna was discovered on it.
They find they can't get the microorganism to react to anything, not even the standard shit they normally like (like fungi w/e). They only discover through POOR HYGIENE (yeah way to go nice one scientists) that the second they come into contact with dead cells they begin to replicate and reproduce at an alarming rate using the 'biomass', for lack of a better word. (thanks system shUFO Afterlight) Experiments obviously continue along this line blah blah yadda yadda yadda. Through the course of experimentation they find that the microorganism does FUCK ALL when in close proximity to either Marker (alien or man(they are the same remember, also beyond this point the black marker is never mentioned again so The Marker is now the red one, but again it doesn't matter). It's learned through some serious leaps of faith and logic that The Marker (black, I lied sorry last time) was designed to supress these, as stated above.
*timewarp* Bad hygiene/something goes wrong w/e, it's not explained. Infection breaks out but is mostly contained. Two scientists get infected by the Necromorph DNA. They are put into quarantine and studied, only one is referred to again. This guy undergoes some changes (becomes necromorph) and upon making eye contact with a human, uses its 'super strength' to break out of an air vent in the containment chamber to escape. Henceforth, for this SOLE reason, they only travel around via air vents for the entire game. This is the SOLE reason I can't stress that enough and how shit the setup is for that. Anyway it goes tits up blah blah blah, people report blockages in the vents, it's the necromorphs/biomass blah blah blah. (If you hadn't worked it out yet, all this shit is happening on the colony that Ichimura is cracking present game).
Fast forward time and Ichimura arrives, and there's a MAJOR break up of plotline here. Apparently this colony sets up for 3 years to prepare for planet cracking (basically smelting entire planets), and Ichimura arrives to do the cracking. This makes sense. What DOESN'T make sense is apparently all this shit goes on on the surface and nobody seems to be aware of it at all in any way, shape or form. It's possible the 'shit hit the fan' after The Marker left the colony, and that would make sense as the Marker is designed to prevent necromorph growth. What doesn't make sense is how either the colony, ichimura, or both are both infected when one of them always had The Marker at some point. This just doesn't make sense and it's never explained, which I will make MORE CONFUSING below.
Forget the colony now, it's unimportant and never comes into this again. On the Ichimura (I'm going to assume shit hit the fan after The Marker left the colony) The Unitologists think they have found another Marker, not realising this one is man made. Kendra(?) your blatant fuck buddycrewmate basically tells you this, she's a typical undercover government evil bitch. W/e fuck her she's not important at all. She dies btw. (that's not a spoiler EVERYONE dies(wait that is a spoiler GUESS I BETTER PUT IT IN SPOILER TAGS)). Anyway people start to go weird and hallucinate, because now apparently the marker DOESN'T inhibit the growth of necromorphs, it actually makes people go crazy. (told you it gets weird). Dr Whatshisname murders the Captain 'by mistake' and pretty much the entire crew, armed only with a syringe (later he upgrades to a knife!!!). He experiments on them with his crazy necromorph idea and CREATES the necromorphs himself (I guess since it's a DNA code that self replicates this is SOMEWHAT plausible.... Yeah fucking right. And I ranted about the FEV too, this takes the biscuit. Spoiler: He dies too, but that's not a spoiler everyone dies. First chapter letters guyssss. Biomass takes over ship, System Shock 2 blah blah blah. Note the circumstances of The Marker and the biomass and the necromorphs is IDENTICAL in everything but name to SS2, replace necromorph with annelid and microorganisms with worms. The whole worshipping it, going crazy, experimenting on the crew to create new lifeforms, etc etc etc. Very very very SS2. If you 'got' SS2, you'll 'get' this bit.
Fast forward to present day. There's some backstory about how youIsaac (the game makes many, many, many attempts to FORCE you into believing you are watching a guy not playing him, even though it tries to make you 'be' him without the whole interface thing...very weird) fucked Nicole for 2 years prior to her getting the commission with USG. It's not important. What IS important is that tape you were watching at the start, which you have watched MANY MANY MANY MANY MANY times (here's where it fucks up severely) has Nicole kill herself. Apparently, despite watching this so much, neither you nor your fuckbuddyknow this. Yet at the end of the game Kendra DOES know this, and tells you to watch it all. Um...what?
Now here's what I don't get. Nicole is dead. Nicole is a necromorph. These are facts. The necromorphs DO NOT WANT to 'be supressed' (evidently, play it and it's obvious) but apparently they DO want to be supressed because they want to be 'made whole again' with The Marker. The Marker supresses them. You know this, it's fact. The game TELLS YOU DIRECTLY WITHOUT QUESTION it does this. And yet in the presence of The Marker I fought the toughest necromorphs in the game, including the hive mind!!! I can't fucking explain this. Don't ask me. It makes sense perfectly. What you're doing makes UTTER sense...if Nicole wasn't fucking in the game. The physical hallucination of her which can manipulate doors, die, and operate computers (and fly spaceships) is simultaneously a hallucination....which Kendra can see and hear....and a necromorph, without the physical traits of one. I just cannot explain or understand this.
Anyway, end game: instead of supressing the hivemind, you KILL the hive mind. This 'fucks up' all necromorphs and turns them from mindless bloodthirsty uberzombies....into mindless bloodthirsty uberzombies. Short story: Necromorph Nicole is on your ship (or is she a hallucination?!) and she's coming back home with you buddy. But she's gonna wait for Isaac to do his 'outro' sequence before she appears....even though she's crouching right underneath him.
For the record a 'Dead Space' is the space around The Marker in which the necromorphs cannot grow/mutate/replicate/reproduce/whatever.
For the record, in a worse way. Isaac's father went missing at a young age. Isaac is 'searching for his father' but never found him. Isaac's father went missing without a trace and his personnel file is locked away and security encrypted by the government. You know what this fucking means. Also the unitologists spaceships are never mentioned more than a few passing comments, so that will be in DS2 as well. I'm done.
edit: Laue just made me figure it out. The 'crazy hallucination' is telling him to take it to the planet to supress them, not the necromorphs. There's constant talk (and an entire character in Dr Kynes) that have 'great ideas' based on craziness.
I must rewatch Event Horizon to see how similar the game was in the end, btw.