From the introduction:
Rapture. The cracked jewel of the deep seas. Eight years ago, 1960, it was brought to its knees when a mysterious individual called Jack came down from the surface and murdered the two most powerful men it knew. Andrew Ryan, creator of Rapture, and Frank Fontaine, the pirate lord of its commerce. After that, Jack had all of Rapture at his fingertips. All the ADAM and Plasmids he could ever desire. But instead he did the unimaginable.
He healed most of the Little Sisters in the broken city and took them back to the surface.