Totally crazy!
When you enter this city you will soon find out that something has went very wrong and it's up to you to save it yay. Anyway the enemies you encounter are called splicers and need a (drug?) of sorts called ADAM and the only way they (and you) can obtain it is by getting it from the little sisters throughout the game. However these girls arent alone, they are guarded by these massive giants in scuba suits called Big Daddies and protect them from harm. If you are lucky enough to kill one of the Big Daddies then you can either choose to harvest or save these little sisters. It's a question of what you think is morally right. Harvesting these girls will kill them, but you get a ton of ADAM for doing so, on the other hand you can save them but only obtain a small amount of ADAM and you can obtain a achievement if you choose to save them. The environment is one of the awesome and (twisted?) places that i've seen in a video game since silent hill. It has a creepy side to it but at the same time it has a side to it that can pull you in and make you feel safe somehow.
As for the main character Jack, you have the option of customizing him and making him stronger by spending ADAM on plasmids and combat tonics which are like your Intelligence, Strength, Sneak etc. in RPG games.
Plasmids are really interesting, they allow you to have superpowers if you will, and either burn, freeze, shock your enemies or you can use your mind and send objects flying toward your enemies. Other cool and fun plasmids to use include the ablility to send bees from your arm and having them attack your enemy, but my personal favorite is being able to take control of a Big Daddy and make them think you are a little sister. Anyone would is thinking of getting this game don't shy away from it, you won't have any regrets in buying this awesome game.