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The story plays out primarily in radio communications and audio diaries, so you're only picking up snippets of the story at a time. Still, those snippets are enough to fill you in on the citizens of Rapture flirting with godhood, the subsequent need to exploit one another, and Ryan's desperate attempts to restore order. It's pretty twisted. But that's not really where the creepiness lies.
The nameless you enters the scene, and immediately you inject yourself with a "gene tonic" with which you can ignite things by snapping your fingers. And yes, it's a little odd that you immediately choose to inject yourself with a foreign substance, but that gets explained sufficiently later. Rapture is a mess. The people are all insane, the lighting is dark and flickery, there are leaks everywhere, and the groaning of metal under the sea or the crazed ramblings of the inhabitants can really freak you out. This game is built to freak you out, and it does a good job with that.
In order to "purchase" more gene upgrades (which you realize quickly you need in this madhouse), you require Adam. Adam is in these undersea slugs, which are implantd in little 10-year-old girls, called "Little Sisters" (later on it is explained why it has to be little girls). You have the choice of saving the little girls from the influence of these slugs and getting your Adam, or you can pull the slug out of them, killing them, but getting you twice the Adam. I chose to play the way that would let me sleep at night, and I ended the story on a happy note, having saved all these little girls. Later on I may play through, killing them all and seeing how it changes the ending. Maybe not. I know it's just a game, but that's just wrong.
One of the more interesting facets of the game are how you can use the environment to your advantage. You can hack the security cameras and security turrets to do your bidding, which makes it a lot easier to get around (if a security camera spots you, it'll send little gunbots after you... if you hack it, it sends those same gunbots after the badguys who wander into its gaze). Hacking is a fun little minigame in which you try to connect tubes from one end to another, only given a few different pieces to work with. It's challenging, but fun and rewarding. And that does lead to a lot of options: In other reviews I've read, the reviewers discuss the different ways they killed the first boss (one of the genetic surgeons who has gone all kinds of crazy). Some led him into the water in a lower level of his area and electrocuted him. Some shot the canisters of gas near him and blew him up. Some set him on fire. I personally used a hacked turret and camera and had robots do all my dirty work.
Between the mad ramblings of the lunatics down in Rapture, the creepy lighting and sound, and harvesting the Little Sisters, this is a really creepy game. After a while you get kind of used to it, and then things are built to creep you out more (like when the lunatics figure out it'll be easier to kill you if they pretend to be one of the many corpses lying around, and then jump up right when you walk up to them). Y'know, if I'm playing a first person shooter, and I'm killing aliens or robots or criminals or enemy military, I'm OK with that. Going through this place that looks like a riot happened in an underwater asylum, not so much. I like the tenseness and excitement that comes from just making it through the enemy onslaught, but being creeped out and feeling like I'm putting these people down because they're just too sick to live.
So it's a good game, with a lot of potential. It allows for a lot of replayability, since you can try different tactics on your enemies. It's got a deep and engaging story, with some pretty significant and dynamic and surprising twists. But it's really freaky.