BioShock 2 is a first-person shooter. The player assumes the character of Subject Delta eight years after the events of the BioShock.[4] The player explores Rapture and fights off Splicers using weapons, environmental hazards, and plasmids.[10] Plasmids grant powers such as telekenesis and must be recharged with "EVE" before further use. The player can equip tonics—passive bonuses varying from increased speed to reduced EVE usage.[9][11] Several weapons in BioShock 2 are unique to the Big Daddy, including a powerful drill and a rivet gun. Unlike in Bioshock, the player can use each weapon in a close-range melee attack, and may simultaneously equip a weapon and a plasmid. If the player is killed, they are revived via "vita-chambers."[12]
Players scavenge ammunition and supplies as they explore Rapture, and may purchase items at vending machines scattered throughout the city.[13] Weapons can be loaded with various types of ammunition for greater effectiveness against certain types of foes.[14] They can also be upgraded at certain machines; three cumulative upgrades enable a special weapon effect—for example, the modified rivet gun shoots super-heated projectiles that ignite enemies.[15]
Some game mechanics were streamlined from BioShock, such as hacking into security systems and turrets. This is represented by a lengthy Pipe Mania–style minigame in BioShock, but a shorter timing puzzle in BioShock 2. The player stops a needle on a colored gauge; stopping on the gauge's green area completes the hack, and hitting small blue areas gives a bonus. If the needle lands on the white or red portions, the hack fails and damages the player or triggers an alarm.[13][16][17] The research camera mechanic was also altered, recording video instead of stills. Using it while damaging enemies in creative ways gives players bonuses or new abilities.[15][18]
ADAM is used to upgrade the player's plasmids and tonics. Upgraded plasmids bestow additional abilities for more tactical usage; for example, an upgraded Electro Bolt plasmid inflicts chains of electric damage between enemies, rather than affecting only a single target.[9] As a Big Daddy, the player can defeat other Big Daddies and either adopt their Little Sisters—having them gather ADAM for the player—or harvest the Sisters for their ADAM outright. After adopting a Little Sister, the player escorts her to corpses littered around Rapture and must protect her while she harvests more ADAM.[14][19] Once the Little Sister has collected enough ADAM, the player can choose to either harvest or save her. Harvesting or saving Little Sisters will provoke attacks from Big Sisters.[20] ADAM can also be collected from slugs when venturing into the open ocean surrounding Rapture.[15]