Some spoilers ahoy.
Where Andrew Ryan controlled the "aristocracy," Frank Fontaine, a con man, began to build a business empire to rival Ryan’s grip, especially by providing for the working class. Around this time, sea slugs with amazing regenerative qualities were discovered by an untrained but genius scientist named Tenenbaum. Forming a possibly romantic alliance with Fontaine, they waged economic war by selling an addicting extract of the slugs called ADAM.
ADAM fuels genetic improvement, but also created addiction as those who go without it start to deform. To harvest back the ADAM from dead junkies, Tenenbaum created Little Sisters, protected by Big Daddies, to reclaim and resell their product. Somehow a prostitute named Jasmine Jolene, bearing Ryan’s illegitimate child, sold the fetus to Tenenbaum, who applied new types of genetic and behavioral engineering on young Jack Ryan at her Little Sister creation facility.
Ryan discovers this betrayal, and kills Jolene in anger. Fontaine has Ryan’s son endure experiments and training to become a weapon against Ryan, and in 1957, sends him to the surface world to be hid. By now, Fontaine and his ADAM-addicted army of splicers and Ryan’s faction have gone into full-blown civil war. Desperate, Ryan takes over Fontaine Futuristics, his splicer factory, to arm his own splicers with more powerful plasmids. Fontaine fakes his own death, and creates Atlas, a revolutionary hero. His armies continue to resist Ryan until New Year’s Eve of 1959, when Jack Ryan returns to Rapture. It is here that the game begins.
Bioshock, it must be said, is an absolutely stunning game.