role-playing game customization and stealth elements, and is similar to System Shock 2. The player takes the role of Jack, who aims to fight his way through Rapture, using weapons and plasmids (genetic alterations), in order to complete objectives. At times, the player may opt to use stealth tactics to avoid detection by security cameras and automated turrets.[26] While exploring Rapture, the player collects money, which can be used at various vending machines to gain ammunition, health, and additional equipment.[27] The player also comes across spare parts that can be used at "U-Invent" machines to create new weapons or usable items. Cameras, turrets, safes, door locks, and vending machines can all be hacked to the player's advantage, providing benefits such as turning on the player's foes, revealing their contents to the player, allowing entry to locked areas, or allowing the player to purchase items at a discount.[28] Hacking requires the player to complete a mini-game similar to Pipe Mania in a limited amount of time.[29] The player is given a "research camera" early in the game, allowing Jack to take photographs of enemies to help analyze them, with better quality photographs providing more beneficial analysis. After performing enough analysis of an enemy, the player is granted increased damage, gene tonics, and other bonuses when facing that type of enemy in future battles.[30] Glass-walled "Vita-Chambers" can also be found throughout the game, which the player does not use directly. Instead, should Jack die, his body is reconstituted at the nearest one, retaining all of his possessions, but only a portion of his full health.[31] In a patch for the game, the player has the option to disable the use of these Vita-Chambers, such that if Jack dies, the player will need to restart from a saved game.
The player can collect and assign a number of plasmids and gene tonics which grant Jack the ability to unleash special attacks or confer passive benefits such as improved health or hacking skills. "Active" plasmids—those that are triggered by the player such as most offensive plasmids— require an amount of the EVE serum to be used in a manner similar to magic points; EVE can be replenished via syringes.[32] These plasmids also alter the player's appearance to reflect "sacrificing one's humanity".[33] "Tonics" are passive plasmids and require no EVE to gain their benefit; the player can only equip a limited number of plasmids and tonics at any time.[34] Tonics can increase Jack's strength and resistance to damage or make hacking machines easier. The game encourages the use of creative combination of plasmids, weapons, and the use of the environment.[35]
A Big Daddy defends a Little Sister from two Splicers, while the player watches.
Plasmids can be collected at certain specific points around the city throughout the storyline, but most often are purchased by the player at "Gatherer's Gardens" using the ADAM mutagen they have collected from Little Sisters. In order to collect the ADAM, the player must first defeat the "Big Daddy"—genetically enhanced humans grafted to an armored diving suit—that accompanies and guards each Little Sister. After this, the player has a moral choice: either to kill the Little Sister to harvest a great deal of ADAM, or to save the Little Sister and gain a smaller amount, though for every three sisters spared a gift of a large amount of ADAM is given to the player. While both choices have their advantages, this element of conflicting morals has an impact on the storyline, and, among other things, on the difficulty of the game itself.[36](ignore the numbers it of wiki)