Humanity has managed to colonize other worlds. Nowhere specific is mentioned in either movie or book but its of dialogue point at there being at least some form of faster-than-light-travel at play (since otherwise we'd only have colonies in the solar system and ships going out into wherever.
At some point there was World War Three, a series of ecological disasters, or something that's more or less axed most of the animal life in the word. Either to try filling the niches left behind to keep the animals that didn't die out in check, aesthetics, or whatever there exists an industry that has been able to create totally life-like critters. Unsurprisingly this technology has advanced from making fake animals to fake people. These Fake People, Replicants, are marketed to would-be colonists (helpers, companions, etc), Military (front line troops, hazardous material workers.) At some point in the past these replicants rebelled, and since they look so much like 'natural' humans they aren't allowed on earth anymore.
Killing Replicants is not called Murder. It is called Retirement. This is very telling since right there in the opening of the movie you're told that Replicants are not considered human. This is reenforced when you get Deckard's 'Replicants are like any other machine' song and dance. Replicants are given the crap jobs in the colonies. The newest model is almost impossible to tell from a normal person even by being around it so they gave it a four year lifespan.
It should be noted that neither book nor movie goes into any of the earlier generations. We don't see much of what went on before. However in the book we do see something of an enforced collective guilt trip humanity has to endure in the form of an engineered series of images, sensations, and the like that they have to go through on a regular basis, presumably so everyone will understand that inflicting pain on eachother is inherently a Bad Thing. In the book it's almost like a religious experience. One that Replicants cannot feel because either by accident or design they cannot feel empathy the same way most people do.
Maybe this was outright stated in the book but I forget. Besides the movie doesn't really go into it. However it is my personal feeling that either the lack of empathy was engineered so that people could point and go 'look they clearly aren't human so we can abuse them all we want' without feeling guilt or it was something found in the process that couldn't be corrected and Tyrell Corporation (Rosen in the book) simply took advantage of the situation and gave people an 'other' group they could safely degenerate while being guilted into tolerating other people.
OK say the world is kindof teetering on the edge of enviromental collapse. You have corporations that are as powerful as governments. You know the whole cyberpunk shebang.
No I'm not trying to talk about what happened on the news last night. Our world's bad off environmentally but the depictions in Blade Runner tend to be along the lines of 'we screwed up bad now it's all a downhill spiral and the only way out is the colonies.'
The Movie's message was so far as I could tell is 'Don't go about shooting people because you're told to', or possibly 'Humanity is what you make of it.'
I dunno really. Even if the group of replicants in the movie were just people trying to get away from something bad they are collectively guilty of several murders, theft, breaking and entering, and a few other things. Granted they are at that point slaves trying to gain freedom (not only freedom from whatever situation they were in, but free from the four year lifespan they were shackled with) but even so we cannot sit and look at these people like they are as pure as driven snows.
About this whole 'Humanity' thing...
Well one of the advantages of the voice-over (which I really wish they'd included as an audio track in the Final Cut) was we got to get in Deckard's head a little. For those not aware there's like five or six versions of the film. The Voice-Over was only in the Theatrical release and there's been rumors going around more or less since the movie first showed that Harrison Ford intentionally made a flat delivery of his lines in hopes it wouldn't get used. Ford has always denied this and stated he did the best he could with what he had.
Granted I'm in favor of the voice-over (with a few cuts here and there such as either outright omitting or at least changing the place of where Deckard goes on about the 'tears in rain' moment so it doesn't absolutely murder the scene but I can understand why it's generally not well recieved. I like it because we get little nuggets of information we couldn't have otherwise with a movie without basically derailing every five minutes. Deckard's bored delivery, if we are to take Ford's assertion he was doing the best with what he had, is a sign he's either walled himself off emotionally, or he's just going through the motions of life and is terminally depressed; I can't make my mind up which.
Compare and Contrast Deckard's flat depressed/emotionless limping along through his life with Roy Batty, leader of the Replicants that Deckard has been ordered to Retire. The two are basically opposites. Deckard just kinda muddles through where Roy grabs Life by the horns, Deckard's romance is awquard and could almost be looked at like rape and Roy's is tender almost loving or at hte very least gentle. Yet where Roy Batty is seen as more 'alive', we also see Deckard as actually having at least a little more care than he does.
How? Sure Deckard firing in a crowd would at first look like he's just taking shots in the dark and hoping to hit the mark, but he isn't firing blind and only fires when he has presumably what he feels is a clean shot. Contrast with Roy Batty trying to get information by going after a guy in a lab set presumably at insanely cold temperatures by ripping his suit off. Not threatening to, not explaining if he doesn't get What He Wants To Know he'll Do Things. He just has his right hand man rip the guy's suit off in such a way that it's impossible to put back on. Could be argued 'Well what's Roy going to do? He's not even considered human. People will shoot him on sight.'
True, but later in the movie his interaction with Eldon Tyrell is telling. The old Man Tyrell is more or less the head (if not founder) of the company that created Roy. There's a definate Father/Son vibe there. They talk, it's clear both are intelligent and have a grasp of interesting concepts, but in the end Tyrell can't give Roy what he wants. So Roy grabs the man's head and crushes it in his bare hands and there's a throwaway line later in the movie when the murder is reported over Deckard's police radio that the guy that was letting Roy and another Replicant hide was also found dead.
So while Deckard is numb to life he at the very least shows restraint. Of course Roy has some excuses here in that he was grown specifically as a soldier and he's only about four years old without any real means of coping with essentially being told 'no, you gave up Everything. You threw away your chance at living in some backwater in piece and it's all for nothing. You Are Still Going To Die.'
Still. Shows neither side as explicitly Saintly, which is always a good thing.
Both Batty and Deckard have a sort of love interest that follows them. Roy's is with him the whole time and helps by finding him a place to hide and plan and get him access to what he needs (Tyrell.) Deckard's love interest happens partway through the movie in the form of a secretary he is told by Tyrell to test to show off how the Replicant Test works. It turns out she's a Replicant that's been given a set of memories so when something emotional happens (like in Roy's case where he finds out he's still going to die) they won't completely flip their lids.
This is actually, in my mind, handled quite well. See Rachael doesn't know at the start she's a Replicant. When she finds out and her Boss/Maker/Uncle (she's patterend off of Tyrell's Niece) won't tell her anything she actually has a somewhat intelligent plan instead of throwing a temper fit (or maybe she threw one off screen and we just didn't see it.)
She goes to find Deckard. OK fine not smart at first because as Rachael herself puts it, "I'm not in the business. I am the business." So why go after the guy that's sent down to hunt down the things she thinks she is? Let's start with the fact she isn't dead yet. I've no idea how the process usually goes, but my assumption is as soon as the little machine tells the Blade Runner that the subject is a replicant their response is to tell the person they can go and as soon as they turn to leave pull out gun and shoot said person until they stop twitching.
At this point neither Rachael or Deckard know eachother, so she has to assume that Tyrell is protecting her, or at least had up until that point. We don't know if she went back later in the movie or once she caught up with Deckard she ended up staying in his apartment (where's the last place you'd find a replicant? A Blade Runner's apartment.) In the movie we get Deckard's boss say they went by Tyrell's and Rachael had run off. This could mean she stayed at the apartment, or just as likely Mr. Eldon Tyrell didn't want his one of a kind experimental Replicant to get waxed because of some silly law so lied. That or he explicitly told her, possibly in a flippant offhand manner, to go find Deckard.
Then she ends up saving his life partway through by killing somebody that was literally strangling the life out of him and was about to make his head go pop. Litle meek secretary girl picked up a gun and not only hit the general area where her target was instead of blowing a hole in Deckard's arm (judging by the hole the gun left when it did fire I'd say it would have shattered the bone) but also went clean through the guy's head. She got the shakes after, but that was the first time she killed anyone. Even if you're good at target shooting there's a world of difference between shooting a bullseye and shooting with the intent to kill.
In spite of basically having been pegged as sorta useless in the official files Pris is resourceful, athletic and competent in her own ways. After all we first see her hanging out Right At The Apartment of one of the guys that could get Roy and Company in with the head of Tyrell Corporation. Some would see this as dumb luck, but I disagree. We see this 'chance' meeting happen after Roy's cold blooded torture of the cryo-lab guy so they know who their target is, and presumably where he lives. All Pris had to do was play scared street rat and string the guy on a little. Doesn't take rocket science, but still it takes initiative and guts to try scamming the guy. Sure Roy might have been waiting a bit away in case things had gone wrong, but even if you have the whole lonely hard up guy stereotype managing to get somebody to not only let you stay at their place after literally running into them and letting one of your friends in takes a kind of skill I'm not sure many have.
When Deckard comes for her she's given an opportunity to run in that she knew he was on the way. Instead she hid, managed a pretty good ambush and actually managed quite well for what little fighting experience she had to work with and the fact her opponent had a gun.
I'm not even going to bother with figuring out an answer to the thirty plus year question of if Deckard is a replicant or not. Doesn't matter. Even the production staff can't agree. A couple of the writers had pegged him as human. Ridley Scott has him as a Replicant. Replicant or Not Deckard goes through the movie gaining a little bit of humanity. He and Rachael leave the ever dark and rainy city (to be fair it's monsoon season and the movie takes place in a handful of days.)
So what about a followup? There were books written after the fact that went into details. Honestly you can take those books however you like, but the movie is it for Deckard and Rachael's story. Theres more to see in the world, but any question left hanging should be left as is. We have too much other stuff we could explore instead.
Which I guess is the whole point of this post. Awhile back there was talk of a sequel to Blade Runner. I talked about it then. I just don't want to see a retread story.
Just as a bit of shameless advertising If anyone wants a copy they can find it on Amazon.
Or if you want the DVD.