The first details about BioShock 2: Sea of Dreams, have been announced a few days ago:
The player will take the role of the first Big Daddy ever created in Rapture, who has seemingly survived the events of the first game. As such, the Big Daddy's drill and rivet gun will be weapons that the player may use, but will overheat if used constantly. Alternatively, the player may also use plasmids, and their effects have been changed, while "upgrade trees" will provide unique ways to upgrade plasmids. Further Big Daddy abilities found in the first game, such as performing a body slam, will also be an option.
As a "rogue" Big Daddy, players will attempt to capture the Little Sisters from other Big Daddies who roam Rapture. Once caught, the player will again have the option of choosing the fate of the Little Sisters. Harvesting remains a possibility in order to gain more ADAM, but rather than rescuing the Little Sister, players will instead be allowed to adopt them. With this option, Little Sisters will ride around with the player's Big Daddy, and will look for ADAM to harvest from corpses; in a manner similar to the final area of the first game, the player will then need to ensure that splicers will not harm the Little Sister as the harvesting process takes place. Rescuing, while not initially an option for the player, will become available at some point in the game.
Returning features from the first game include Vita-Chambers to allow the player to regenerate, and audio diaries. In addition to the single-player story, online multiplayer will be a feature in BioShock 2.
Plot:
Seven years after the events of BioShock, a number of towns around the world have been reporting kidnappings of young girls by attackers described as "thin", with red lights often reported as either being worn by the attacker or spotted nearby in the sea. This attacker — the Big Sister, one of the Little Sisters from the first entry in the series who has become mechanised in a similar fashion to the Big Daddies — has become the ruler of the underwater city of Rapture; Tenenbaum, discoverer of ADAM and original creator of the Little Sisters, states that the Big Sister has begun kidnapping girls and turning them "into creatures... like her".