The main focus of the gameplay is exploring Rapture, fighting off the Splicers that attack in great swarms and seeking out the Little Sisters roaming the world with their Big Daddy bodyguards. As in the first game, if you defeat one of these protectors you can either harvest the Little Sisters and take their ADAM, or protect them. This time, if you choose the latter then they will lead you to corpses from which they can gather yet more ADAM. This process attracts waves of vicious Splicers, so you'd better be prepared for a fight.
Once again, you can spend the ADAM on Plasmids, which are genetic modifications allowing you to emit bolt of lightning, freeze your enemies, shoot swarms of bees and much more. Artfully combining these with weapons, such as the giant drill attached to your right hand, is the secret to success. You can also use them to interact with the game world, electrifying a pool of water to create a deadly trap, for example.
There are plenty of other tricks to make your journey through Rapture easier. The research camera from the original BioShock can now capture video, and you must damage the framed enemies in creative ways to unlock research bonuses. Hacking has also been revamped, with a skill meter taking the place of the pipe-arranging mini-game of the original. You have to stop a moving bar in the green area to complete the hack, while the blue areas offer bonuses and the red areas set off the security alarm.