Rapture was built in the early 50's by Andrew Ryan, a businessman dissatisfied by the politics of post-WWII world. There, he invited the "world's best and brightest" to form a city without God, government, morality or ethics. Everything changed when ADAM was discovered, a substance that rewrites your DNA and gives you superpowers. This discovery led to the rise of Ryan's rival, Frank Fontaine, the creation of the armored Big Daddies and creepy but innocent Little Sisters. Ultimately, Fontaine staged his own death and was reborn under the alias Atlas, waging a war with Ryan that would destroy Rapture.
But that was in 1959. One year later in the first game, and nine years later in the second game, you come across Rapture. You will need to wield deadly weapons, such as the Drill, the Spear Gun and... the Hack Tool or Research Camera. All these can be upgraded and many more (with the exception of the last two) by adding firepower, bigger rounds, new types of ammo and finally upgrading them to the final level. Upgrades include bullets that catch on fire, electrifying buckshots and the ability to spin your Drill and make bullets ricochet in every direction!
Not to mention the Plasmids, abilities gained from gene-altering such as shooting bolts of electricity of fire from your fingertips, creating whirlwinds, letting loose swarms of bees, creating fake copies of yourself to trick the enemy, exiting your body and entering a ghost-like state and manipulating Security Cameras. Not to mention the Gene Tonics, passive abilities that allow you to hack computers faster, deal more damage with certain weaponry or enhance your armor.
I'm in love with this game...
But how do you get the ADAM needed for this? By harvesting or rescuing Little Sisters that hold ADAM inside them. But this is a moral choice and harvesting/killing them will give you more ADAM, but it's... well, bad. So it's your choice, be sure to make the best one.
Oh, one more thing. In the first game you play as a human, but he's a "wet rag" in the words of the narrator of BioShock 2's trailer. In the second one, however, you play as a Big Daddy, so have fun with that Drill!