The underwater-city Rapture has further deteriorated, with water breaking in everywhere and whole sections flooded by water. But as bad as the situation is, the city still isn’t empty. After Andrew Ryan’s death, his long adversary, the psychiatrist Dr. Sophia Lamb took over the helm – embracing her own cult, making the mutated splicers that roam the city her worshippers. In the middle is Delta, the first of the protectors of the Little Sisters better known as Big Daddies. Eternally bonded to one and only one Little Sister called Eleanor, daughter of Sophia Lamb, he’s damned to search the city and find her or die trying.
The player slips into the mighty boots of Delta, making his way in a typical first-person-shooter-style through the city of Rapture to find and confront Dr. Sophia Lamb, who holds Eleanor against her will. The gameplay has changed with the introduction of plasmids and weapons that can be used together, allowing for new combat strategies. Players can also use special abilities like "Winter Blast" to freeze enemies or "Scout" to move around undetected. The Little Sisters roaming the levels are still crucial for accessing Adam, but now players have a choice: they can adopt them and protect them from harm, or harvest them themselves. Harvesting a Little Sister requires the player to fend off splicers while it searches for Adam-infused bodies. Once the task is complete, the player can choose to keep the Little Sister as their own or free her.
Another new feature is the hacking mini-game, which allows players to access vending machines and turn turrets against enemies. The mini-game requires precise timing to hit the right fields on a spinning gauge, and success rewards the player with benefits. BioShock 2 also features multiplayer for up to 10 players, developed independently from the single-player campaign. Players can compete in various modes such as Deathmatch, Team-Deathmatch, Capture the Sister, and Adam Grab, while earning ranks and unlocking additional radio messages that advance the story behind the civil war in Rapture.