Rapture is a city-state, located on an island in the middle of the ocean. The city is divided into two sections: the wealthy Upper City, and the more modest Lower City. The two areas are connected by a mysterious network of canals and underground tunnels.

A year prior to the Fall of Rapture, a mysterious infection began to spread throughout the city. The infected became aggressive and sick, and the city's government responded by shutting down the infected and locking them up. Only the people in the upper city were protected, leaving the lower city open to the wild.

Big Daddy is the main protagonist of the game. He is a giant humanoid with the ability to shoot powerful lasers. His true name is Noah, and he was given the ability to shoot as a result of a experiments that went terribly wrong. As a result of the experiments, Noah has been forced to shoot himself and his own children.