It's 10 years since the first game. The Big Sister returns to rapture, bringing back the Little Sisters with her. You play as the original Big Daddy. You can use the drill, the rivet gun, and you can still use plasmids. Little Sisters have been given a new look to be less creepy, as you're seeing them from the eyes of their protectors. You don't get your own Little Sister from the start, as you're a renegade Big Daddy. So you fight other Big Daddies to get your own sister. You can still harvest them, but the other option has been changed to "adopt" instead of rescue. If you adopt the sister, she'll ride on your shoulders, and you can go around Rapture looking for corpses for her to collect ADAM from. When your sister goes to harvest ADAM, a wave of Splicers will come and start attacking to try and harvest her. You have to hold them off until she finishes harvesting. You gain some ADAM from this, but not enough to tide you through the game. You can permanently rescue them, but the article doesn't say how. The Big Sister is one of the surviving little sisters from the last game. She's the new ruler of Rapture, and is the main antagonist of the game. With each Big Daddy you kill, the game lets you know how close you are to incurring the wrath of the Big Sister. When you do, the little sister you're with will sing a song about how the Big Sister doesn't want you playing with her, then you get jumped.