At the beginning of the fall of Rapture, Andrew Ryan brought in a psychologist, Sofia Lamb, to calm the minds of the people. However her intentions were not the same as Ryan's. She advocated a collectivist philosophy which was at odds with Ryan's objectivist views, and she came to be worshiped by some of the denizens of Rapture. She believed in the power of community while Ryan believed in the power of individual minds.
After the deaths of Ryan and Frank Fontaine at the end of BioShock, Lamb became the de facto leader of Rapture. She opposes the old Big Daddy system with her own army of Big Sisters.
BioShock 2 begins with a first person cinematic showing Subject Delta protecting Eleanor, the Little Sister he was partnered with, from Splicers while she gathers ADAM. After defeating all of them, Sofia Lamb arrives and takes Eleanor and forces Subject Delta to shoot himself with a pistol.
Delta awakens 10 years later and embarks on a journey to get back his "daughter", Eleanor, who is actually the daughter of Sofia Lamb. With the help of a gentleman named Augustus Sinclair, Delta navigates several sections of Rapture, using a train to get from location to location.
Eventually, he arrives in Fontaine Futuristics and locates Eleanor. Unfortunately, Sofia Lamb is also there, and suffocates Eleanor with a pillow so that Delta goes into comatose. Delta wakes up strapped to a table. A Little Sister, sent by Lamb, injects him with ADAM, which allows Delta to take control of the Sister. The girl gathers up pieces of armor for a Big Sister and gives them to Eleanor, who puts all of it on and becomes one of them. Delta is freed by Eleanor and together they fight their way to Sinclair's escape pod.
Unfortunately, Sinclair himself is captured and turned into a Big Daddy, and Delta is forced to kill him to get a pass code. Delta hangs on to the escape pod as it rockets away from the city up to the surface, and watches as Sofia Lamb drowns inside the escape pod.
The game's ending can be slightly different or completely different depending on the decisions the player makes during the game. This is dependent on what the player does when he encounters Big Daddies escorting little sisters. If the player rescues all of them, the positive ending will occur. The other endings are reserved for when the player rescues and harvests some little sisters. This is the neutral ending. The evil ending will occur when the player harvests all Little Sisters that are encountered.
