Lamb's reign marks a distinct philosophical shift in Rapture. Ryan's Objectivist view of the power of the individual is replaced with Lamb's Marxist views revolving around the power of community. Lamb's views have spread like wildfire through Rapture, leading to cult-like devotion from some of Rapture's Slicer population. As with any political shift, however, things are not bright and cheery in Rapture (not that they ever were... but you know what I mean). In the midst of Rapture's ideological turmoil, a mysterious new citizen - the Big Sister - begins stalking Rapture.
Rather than playing as an outsider, Bioshock 2 finds you behind the faceplate of a Big Daddy. Not just any Big Daddy, you're a prototype Big Daddy, Subject Delta, who has somehow gained free will and is seeking the Little Sister you bonded to years ago.
Free will plays a major role throughout the single-player experience. As a Big Daddy, you journey around (and outside) Rapture in search of new Little Sisters. Once found, you are presented with the choice of either helping them harvest Adam or stealing it from them, an act no Big Daddy would ever commit. The first game offered a similar choice, though either way the eventual gains added up. Harvesting Little Sisters yielded quick Adam gains, though in the end, you had the same amount if you saved them. As in the last game, offing the girls is a quick, easy way to nab some Adam and will actually make you more powerful quickly, but at a cost. Choosing to save Little Sisters opens up a new mechanic, adoption. Rather than letting the girls run back to their portholes, you can choose to protect them as they continue harvesting. The latter choice is riskier, though it could yield more Adam in the long run.